The 4G data usage noted on my account wasn’t anything major, mind you—just a few kilobytes here, a few megabytes there, often in the middle of the night when my iPhone was asleep on my bedside table, and well within range of my Wi-Fi base station.

Small though they are, though, those odd bits of cellular data usage can really start to add up, particularly when you’ve signed up for a “capped” 4G data plan.

Meanwhile, some iPhone 5 users have complained of even larger chunks of rogue cellular data usage on their accounts—as much as a gigabyte a day, even when they hadn’t strayed far from Wi-Fi.

A quick, over-the-air update of the Verizon iPhone 5’s carrier settings should plug its mobile data leak.

The answer, as it turns out, is yes.

Apple has issued a support notice warning that “under certain circumstances,” the iPhone 5 “may” consume Verizon cellular data even when it’s sypposed to be using Wi-Fi data instead.

The fix? An “over-the-air” update to the iPhone’s carrier settings, which takes just a few taps and only a few minutes of your time.

Here’s how to grab the update:

• Tap the Settings app on your iPhone 5’s home screen, then tap General, About.
• An alert titled “Carrier Settings Updated” will pop up on the screen. Tap the “OK” button.
• Next, power off your iPhone: press and hold the “sleep/wake” button that sits on the top edge of your handset until the “slide to power off” slider appears, swipe the slider control from left to right, then wait for the spinning wheel in the middle of the screen to disappear.
• Press the “sleep/wake” button again; the Apple logo should appear, and you should see your iPhone lock screen a few seconds later.
• Last but not least, double-check that the updated carrier settings were properly installed. Tap Settings, General, About, then make sure the “Carrier” entry reads “Verizon 13.1.”

I just applied to carrier update myself, and I’ll be keeping an eye out for any unusual 4G data usage on my iPhone over the next few days—as should you.

And if you’re still seeing random cellular data charges while your iPhone is connected to Wi-Fi, contact Verizon and let them know, pronto.

Bonus tip

 
You can check your cellular data usage on Verizon by visiting your online Verizon Wireless account, then selecting “My Usage” under the “My Bill” menu heading.

There’s also a “My Verizon” app that’ll let you view your data usage and monthly charges directly from your iPhone.

Looking for more iPhone tips? Click here!

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Greetings, Jed, and glad you asked. First things first: when it comes to cell phones, “throttling” refers to the practice of a carrier intentionally slowing your wireless data speeds—usually because your carrier thinks you’re using too much data.

So, how much data is too much? That depends, of course. Most carriers say they’ll only throttle their heaviest data users, but stop short of setting a hard and fast limit—more on that in a moment.

And what happens if you do get throttled? Well, expect your carrier to restrict your smartphone to 2G data speeds—in other words, think the speed (or lack thereof) of an old dial-up modem. Once your next billing cycle begins, the leash will (generally) come off.

So, what are the respective throttling policies for the big four U.S. carriers? Let’s start with…

AT&T
Since October 2011, AT&T has been throttling the “top 5 percent” of its heaviest data users—but only for those users who have been grandfathered into AT&T’s old unlimited 3G plan, which is no longer available for new subscribers.

Related: Smartphone data plans compared

Since you’re on a “tiered” 2GB plan rather than an unlimited plan, Jed, you don’t need to worry about getting throttled for exceeding your 2GB monthly data limit. That said, you’ll still have to pay an extra $10 for every gigabyte you use above and beyond your 2GB data plan.

For those of you who still are on AT&T’s unlimited 3G plan, recent reports suggest you may be tagged as a “heavy” data user—and thus, subject to throttling—after using as little as 2GB of data within a billing cycle, which doesn’t leave much room for streaming music or video. Don’t like the threat of being throttled? Simple, AT&T says: just switch from an unlimited plan to one of its tiered data options.

Sprint
The last of the four big U.S. carriers with an unlimited 3G data plan swears up and down that it won’t throttle you under any circumstances, although there are some disgruntled subscribers who claim otherwise.

Keep in mind, though, that Sprint reserves the right to “deny or terminate service for any misuse or any use that adversely affects network performance.”

What constitutes misuse? Using your smartphone as a wireless modem without the proper tethering plan would probably qualify, as well as using heavy amounts of data while “roaming” on another network.

T-Mobile
Unlike AT&T and Verizon (which we’ll be getting to next), T-Mobile won’t charge you extra for exceeding your monthly data limit; instead, it’ll throttle your data to 2G speeds until your current billing cycle is up. In other words, throttling is less of a punishment for T-Mobile users than it is business as usual.

Related: Exceeded your monthly smartphone data limit? Here’s what you’ll pay

Verizon Wireless
The “Big Red” carrier claims it doesn’t throttle anyone. Instead, Verizon says, it employs “Network Optimization”—a system that uses “network intelligence” to slow down the data speeds of its heaviest users only “when necessary” rather than for the remainder of a billing cycle.

Well, “network optimization” is one way of putting it; in my book, though, deliberately slowing the data speed of a wireless user counts as “throttling,” even if it’s only done “when necessary.” So let’s just call “network optimization” what it is—”throttling”—and move on.

Anyway, Verizon may throttle any subscribers who are a) grandfathered into its old unlimited 3G smartphone plan (no, 4G LTE users won’t be affected), b) among the “top 5 percent” of data users in a month, and c) connected to a “congested cell site.”

How much data is too much? About 2GB or so, Verizon says, although that figure is subject to change. To find out if your data speeds are being slowed, you’ll need to check for a notice on your bill, or on the home page of your online Verizon account.

Hope that answer your questions, Jed. Have more? Post ’em in the comments below.

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Each of the biggest U.S. carriers has its own overage policies, and it’s pretty much a case of pick your poison.

The sole exception: Sprint, the last of the big four U.S. carriers to offer unlimited (well, more or less—more on that in a moment) 3G and 4G smartphone data.

AT&T
If you’re signed up for one of AT&T’s cheaper plans, such as the $15-a-month, 200MB Data Pro plan (which is on the way out, at least as far as new subscribers are concerned) or the upcoming, $20/month 300MB Data Plus option, AT&T will charge you for another month’s worth of data if you exceed your standard monthly allowance.

In other words, if you’re on the $15-a-month 200MB Data Pro plan and you use 201MB of data in a single billing cycle, you’ll see a $30 data charge—the standard $15 fee, plus $15 in overage—on your bill.

For AT&T’s pricier plans, including the $25 2GB Data Pro plan (which, like the old 200MB Data Pro plan, won’t be available to new customers as of January 22), or the new 3GB or 5G options (for $30 and $50 a month, respectively), expect to pay $10 for each additional GB of data you use above and beyond your monthly data cap.

Related: Smartphone data plans compared: Bargain plans becoming more and more scarce

Sprint
It may be the last bastion of unlimited 3G and 4G smartphone data among the big four U.S. carriers, but that doesn’t mean Sprint will let you go nuts with downloads on your iPhone or Android handset.

Like most carriers (such as AT&T, for example), Sprint has a clause in its wireless service agreements that gives it the option of throttling or even cutting off bandwidth hogs who “generate excessive amounts of Internet traffic.” It’s worth noting, though, that Sprint CEO Dan Hesse claims that “98-99 percent” of users will never run afoul of the carrier’s “terms and conditions.”

T-Mobile
The old “Get More” labels several of its smartphone 3G and 4G plans as “unlimited,” and they are … sort of.

Subscribers to T-Mobile’s Unlimited Plus (2GB of “high speed” data for $20 a month), Premium (5GB for $30 a month), and Ultra (10GB for $60/month) smartphone data plans can, indeed, surf as much as they like without fear of paying any overage charges.

But the devil’s in the details—and in this case, the key detail is “high speed” data.

If you’re a T-Mobile unlimited data user and you exceed your monthly allowance, you can kiss your speedy 3G or 4G data goodbye, as the carrier will “throttle” your data speed to 2G levels (think dial up-modem speed) for the rest of your billing cycle.

On the other hand, T-Mobile’s dirt-cheap, “Simple” plan, which offers 200MB of data a month for a mere $10 a month, isn’t an unlimited plan at all; instead, expect to pay 10 cents for every extra megabyte you use—or roughly double the rate of your initial 200MB data allowance.

Verizon Wireless
The biggest wireless carrier in the U.S. has one rule when it comes to data overages: $10 for every extra GB of data over your monthly limit, period.

Have more questions about your smartphone bill? Let me know!

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And while pricier 3G and 4G data plans do—on paper, anyway—offer more bang for your wireless buck, chances are you’re not using enough mobile data to reap any real benefit.

First, some news. As of Sunday, January 22, AT&T is scratching its $15-a-month, 200MB “Data Plus” plan from its list of options for new customers. (If you’re currently signed up for AT&T’s 200MB Data Plus plan, don’t worry—you can keep it, at least for the time being.)

That leaves T-Mobile as the sole, nationwide U.S. carrier with a sub-$20 monthly data plan for smartphones.

For $10 a month, you can sign up for T-Mobile’s “Simple” data plan, which offers 200MB of data per month.

(Note: T-Mobile’s big U.S. competitors also offer cheaper data plans for as little as $10, but only for so-called “feature” phones, not smartphones.)

The next cheapest smartphone data option for new customers is AT&T’s new Data Plus 300MB plan, which serves up 300MB for $20 a month.

After that, the smartphone plans from the biggest U.S. carriers, including AT&T, Sprint, T-Mobile, and Verizon Wireless go up and up.

Verizon’s cheapest smartphone data plan, for example, goes for $30 a month for 2GB, while Sprint—the last big U.S. carrier with a truly unlimited data plan for smartphones—offers nothing but unlimited data bundles, starting at $80 a month (including a $10-a-month “premium data” fee) for all-you-can-eat data, 450 voice minutes, and unlimited text messages.

Click chart to expand

Of course, the pricier the data plan, the more bang you get for your wireless buck—well, theoretically, anyway.

Related: How big is a GB, anyway?

Indeed, one of the worst values among the smartphone data plans in my little survey is among the cheapest: AT&T’s soon-to-expire, $15-a-month Data Plus 200MB, which comes out to a relatively hefty 7.5 cents per megabyte.

AT&T’s new Data Plus 300MB plan offers a slightly better value at 6.6 cents a MB, but that’s more than a cent and a half per megabyte higher than T-Mobile’s competing $10-a-month, 200MB “Simple” data plan.

And the more you spend, the better the value. AT&T’s Data Pro 5GB plan offers 5GB of monthly data, plus the ability to use your smartphone as a mobile Wi-Fi hotspot (more on that in a moment), for $50—or a cent per megabyte.

Up the ante even more, and the value gets better. Take Verizon’s $80-a-month 10GB data bundle, which comes out to 0.8 cents a megabyte, or T-Mobile’s “Unlimited Ultra” plan, which delivers 10GB of “high speed” data for $60 a month, or just 0.6 cents per MB.

But here’s the thing: getting great value per MB on a 10GB, 5GB, or even 2GB monthly data plan doesn’t mean much if you’re not using all your monthly data.

And according to a Consumer Reports survey from last summer, most of us don’t even come close.

Related: Which data plan should I get for my iPhone?

In fact, the survey found that the average smartphone user on AT&T consumed only about 360 MB of data a month. And when considering only “median” smartphone data use (or the largest group of users in the middle of the range), the figure for monthly AT&T data usage fell to a mere 120 MB, well shy of the 3GB data cap on AT&T’s new $30-a-month, 3GB Data Pro plan.

That’s why it’s worth checking your monthly bill or calling your carrier to see just how much data you’re using on your smartphone each month.

Are you using less than 300MB or 200MB of wireless data a month? If so, you might be better off with a cheaper data plan, even if it doesn’t offer the same value per MB as a pricier option. And if you’re using 2GB or more data a month, then hey—time to cash in on the values offered by pricier data plans.

The good news is that most carriers will let you change your monthly data plan even if you’re still under contract; the bad news, though, is that you might not have many bargain options, particularly if you’ve signed up with Sprint or Verizon.

There’s yet another key variable to consider when it comes to picking a smartphone plan: overage charges, which you can read about right here.

Got smartphone questions? Post ’em below!

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The cheapest 16GB iPhone 4S costs a whopping $649, or a full $450 more than a carrier-locked version.

Meanwhile, the 32GB iPhone 4S goes for $749 (versus $299 with a two-year carrier contract), while the 64GB model sells for $849 (compared to $399 through a carrier). Ouch.

Now, Apple is billing the iPhone 4S as a “world phone” that works on worldwide GSM and CDMA networks.

But on its website, Apple specifies that the unlocked iPhone 4S will not work on CDMA carriers like Sprint or Verizon Wireless.

Related: Why the “unlocked” version of the iPhone 4S won’t work on Sprint or Verizon

The restriction also applies to pre-paid carriers Boost Mobile and Virgin Mobile, which both operate on CDMA networks.

Standard SIM card and the iPhone 4S's smaller micro-SIM, compared

To hop from one carrier to another using the unlocked iPhone, you’ll need a micro-SIM card: a little plastic module that identifies a phone on a given carrier’s network.

You can typically request a micro-SIM card for a nominal fee (as in a few bucks or so) from the GSM carrier of your choice, although you’ll need a data plan (either pre-paid or post-paid) to go with it.

While you can buy an iPhone 4S from a carrier for hundreds less than the unlocked version, you’ll have to sign a two-year service contract to do so.

Also, the handset will be “locked” to the network of the carrier you bought it from, meaning you won’t be able to swap micro-SIM cards to jump from one network to another.

Got questions about the unlocked iPhone—or about unlocked phones in general? Post ’em below.

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Sandra writes: Hi! I am a Verizon customer and really want the new iPhone 4S, but I really want to talk and surf so I am torn between Android and iPhone. In the future, will Verizon update its CDMA network? And if I have the iPhone 4S, will I be able to download software to be to talk and surf? Or will I have to buy a new phone?

Hi Sandra! So, if you’ve been pining for an iPhone 4S on Verizon Wireless but talking and surfing at the same time is a priority, bad news: neither the new iPhone 4S or the older iPhone 4 will ever let you both chat and use data on Verizon’s 3G or 4G networks.

That’s because existing Verizon iPhone models, including the new iPhone 4S, have been designed to work only on Verizon’s older CDMA network, and CDMA cellular networks (and this includes Sprint, too) don’t support simultaneous voice and data calls—not now, and not ever. Update: Not entirely true, see below.

GSM-based networks, however, do let you talk and surf at once—and here in the U.S., that means AT&T and T-Mobile.

Now, Verizon is quickly rolling out its new, faster 4G LTE network, which will eventually let you talk and surf at the same time—but not until Verizon activates its new “voice over LTE” feature, which won’t happen until sometime next year.

Even if Verizon’s LTE network did allow simultaneous voice and data calls right now, it wouldn’t do iPhone 4S users any good. Like I said, the current iPhone hardware won’t work on LTE networks at all, not even with a software update. In short, you’d need a new phone.

Of course, the eventual iPhone 5 might support LTE, but we won’t know for sure until Apple spills the beans.

OK, so what about Android? There are already plenty of LTE-ready Android phone on sale through Verizon, but until Verizon activates its voice-over-LTE feature, none of them will let you chat and surf simultaneously.

Update: A reader points out that there is at least one Android phone on Verizon, the (otherwise trouble-plagued) HTC Thunderbolt, that does handle simultaneous voice and 3G data calls. According to Electronista, however, Verizon has “downplayed” the feature because “the technique used leads to data being much slower than it would be otherwise.” 

In other words, if you want a smartphone that’ll let you both talk and surf, right now, you’re better off switching to a GSM carrier, like AT&T or T-Mobile.

AT&T offers both Android phones and the iPhone, and while T-Mobile still lacks the iPhone, it does offer a range of enticing Android handsets.

For a Verizon phone that will eventually let you chat and browse simultaneously, consider the LG Revolution, the first Verizon Android handset to support the upcoming voice-over-LTE feature. (Other existing Android phones may also be updated to support voice over LTE, but Verizon hasn’t detailed those plans yet.)

Hope this helps—and if you have more questions, let me know.

Bonus tip: Don’t forget that if your Verizon or Sprint phone is connected to a Wi-Fi network, you will be able to chat and surf simultaneously. Once you switch back to 3G, though, your browser won’t work while you’re on a voice call.

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According to a note posted on its online store, Apple will offer an “unlocked” iPhone 4S—that is, a version of the new iPhone that hasn’t been “locked” to a particular carrier’s wireless network—starting in November. Update (11/11/11): The unlocked iPhone 4S is now available on Apple’s website.

It won’t be cheap, though, nor will it work on Sprint or Verizon Wireless.

Expect to pay a whopping $649 for the 16GB version—an eye-popping $550 premium over an iPhone that’s “locked” for use on AT&T, Sprint, or Verizon. The 32GB unlocked iPhone, meanwhile, will go for $749, while the unlocked 64GB iPhone will sell for $849.

Looking for more of a bargain—relatively speaking, anyway? Consider an unlocked 8GB version of last year’s iPhone 4, which Apple is selling for $549.

Why the sky-high price tags? Well, the unlocked iPhone 4S is a no-contract phone that isn’t “locked” to any one carrier—meaning, unfortunately, that you won’t be eligible for any carrier price breaks.

The $199, $299, and $399 iPhone 4S models on sale through AT&T and Verizon come with mandatory two-year contracts, along with generous carrier subsidies to sweeten the pill.

The beauty of an unlocked phone (get the scoop on what “unlocked” means right here) is that you can simply pick one up, insert a working SIM card (or a smaller “micro-SIM” card, in the case of the iPhone 4 and 4S), and start making calls—perfect for those who want to use an iPhone with a non-iPhone carrier (like T-Mobile), or jetsetters who want to use cheap, local SIM cards while making calls abroad.

Standard SIM card and the iPhone 4's smaller micro-SIM, compared

What’s the catch? Well, Sprint and Verizon both run CDMA networks, which are incompatible with GSM networks and don’t even use SIM cards. Indeed, any phone used on Sprint or Verizon must be specifically programmed to work on their respective networks.

Now, the new iPhone 4S is a “world phone” that’s compatible with both CDMA and GSM networks—so, theoretically, you could call Sprint or Verizon and ask them to activate your unlocked iPhone on one of their respective networks.

On its website, however, Apple explicitly states that “the unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers such as Verizon Wireless or Sprint.”

The restriction probably also applies to Boost Mobile or Virgin Mobile, two pre-paid carriers that also employ CDMA networks.

In any case, you can always snag a pre-paid micro-SIM card from AT&T, T-Mobile, or any number of other pre-paid carriers carriers (they’re available for a buck and up on Amazon), plug it into your new unlocked iPhone, and start chatting away.

A couple of quick caveats about using 3G data on an unlocked iPhone:

• You’ll need a data plan associated with your micro-SIM card to surf the web, stream music, or otherwise consume cellular data on an unlocked iPhone. If you’re using a post-paid micro-SIM card and you start surfing away without a data plan, you may get hit with exorbitant data fees, so easy does it. Another option: skip the data plan and only use Wi-Fi data. To do this, tap Settings, General, Network, and then flip the Cellular Data switch to Off.
• While you can access data on T-Mobile’s network through an unlocked iPhone, the iPhone 4 doesn’t support the proper frequencies for T-Mobile’s speedy 3G and 4G networks. That means you’ll have to settle for the slower EDGE network, which is OK for email and light web surfing but a non-starter for streaming music or video.

Got more questions about the unlocked iPhone 4S? Sure you do. Leave ’em below, or shoot me an email.

Note: I’ve updated this story to include information and pricing for the new, unlocked iPhone 4S and the older iPhone 4. 

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That’s bad news for those of us who don’t send and receive, say, 100 messages a day, and based on a quick survey of the post-paid messaging plans offered by the other big carriers, unlimited messaging plans are increasingly becoming the norm rather than the exception.

In other words, the days of getting monthly “buckets” of, say, 200 or 500 messages seem to be numbered.

The scoop on AT&T’s “streamlining” of its messaging plans comes from Engadget, which confirmed the news with AT&T and adds that the changes will take effect August 21.

AT&T also promises that current customers will get to keep their existing messaging plans (including the soon-to-be-extinct $10-a-month plan for 1,000 messages), even if they decide to change phones.

The other big carriers also seem eager to sign us all up for unlimited text/picture messaging plans, while at the same time weaning us off unlimited data for our smartphones.

So, what exactly are our messaging options for the big four U.S. carriers: AT&T, Sprint, T-Mobile, and Verizon Wireless?

I surfed around the various carrier sites and culled a few facts and figures, with the chart below taking AT&T’s upcoming plan changes.

First of all, check out the prices for paying per message. Twenty cents each, both to send and receive? Yikes—and that’s just for text messages, with AT&T charging as much as 30 cents per picture message.

Also, limited buckets of messages are becoming increasingly scarce. AT&T and T-Mobile don’t offer them at all, while Sprint and Verizon only offer a couple buckets each. (Verizon Wireless’s $20-a-month, 5,000-message bucket is intended for those on family plans, by the way, while individual lines can get unlimited messaging for the same price.)

The biggest bargain? T-Mobile’s $10-a-month unlimited messaging plan for individuals. Verizon Wireless, on the other hand, will only give you 500 messages for $10 a month.

While Sprint and T-Mobile will sell your separate individual and family messaging plans, they’re also seriously hyping their unlimited bundles (which include different combinations of voice, data, and messaging), with each carrier offering bundled unlimited plans starting at $49 a month.

Overall? Like it or not, the big carriers seem to be pushing us all toward unlimited messaging plans—and hey, that’s great for those who send thousands of SMS messages a month, but it’s not so wonderful for anyone (like me, frankly) who only needs to fire off a text or two a day.

So, what about you? Do you have an unlimited messaging plan or a bucket of messages—or do you pay per message?

(Photo credit: iStockPhoto)

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Unless you’re on Sprint, there’s a good chance your cell phone carrier has you on a short leash when it comes to speedy wireless data.

Indeed, most carriers now limit, or “cap,” their 3G and 4G data plans at levels ranging from 200 MB to 10 GB a month, and they’ll charge you extra if your creep over your monthly limit. (Not sure how big a megabyte or a gigabyte is? Click here for help.)

But here’s the thing: according to a recent Consumer Reports survey, you might actually be using a lot less smartphone data than you think—and that means you may be paying more on your data plan than you need to be.

Indeed, the survey found that the average smartphone user on, say, AT&T, used only about 360 MB of data a month—a figure that’s skewed by a “small percentage” of data hogs downloading huge amounts of data onto their phones.

When considering only “median” smartphone data use (that is, the largest group of users smack in the middle of the range), the figure for monthly AT&T data usage falls to a mere 120 MB.

That means the typical AT&T subscriber could safely downgrade to the carrier’s $15, 200 MB-a-month “DataPlus” plan with room to spare.

The median Verizon Wireless smartphone user consumed slightly more data than those on AT&T—think 158 MB a month rather than 120 MB, according to Consumer Reports. Meanwhile, median data use on T-Mobile was a mere 48 MB a month.

Surprised? I certainly was, although I figured that my own smartphone data use was probably somewhat above the mean.

Well … turned out I was only half-right. Logging into my account over the AT&T web site, I found that I’d used less than 100 MB of data a month for February and March, and just a little more than 200 MB each in April and May. In June, my iPhone 3G data use slipped to about 150 MB, although it skyrocketed to a whopping 923 MB in July—but only because I’d been performing a little 3G battery-drain test.

So, what does all this mean to you? Well, consider checking with your carrier to see how much wireless data you’re really using each month. You can log into your account online, or just call and check with customer service.

Once you’ve figured out how much data you’re using each month, check out your carrier’s data-plan options.

Using less than 200 MB of data a month? Maybe you should switch to a cheaper 200 MB smartphone plan, provided your carrier offers one. (AT&T and T-Mobile do, for $15 and $10 a month, respectively; Sprint and Verizon don’t; check out this post for more details.)

Source: Consumer Reports

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Why? Because there are two very different flavors of cellular networks and phones in use around the globe: CDMA and GSM, and they’re about as compatible as dogs and cats.

And yes, you guessed it: AT&T and T-Mobile use one of these competing technologies, while Sprint and Verizon use the other.

Indeed, every wireless carrier in the world must choose between either CDMA (short for “code division multiple access”—yikes) or GSM (“Global System for Mobile Communications) cellular technology.

In Europe and Africa, you’ll mainly find GSM networks; in Asia, CDMA pretty much rules the roost.

Here in the U.S., we’re split more or less down the middle, with AT&T and T-Mobile using GSM, while Sprint and Verizon go the CDMA way.

OK, so what’s the biggest difference between GSM and CDMA networks?

The most obvious one is that GSM technology uses a tiny plastic card, called a “SIM” (short for “Subscriber Identification Module”), to identify a specific phone on its network.

An AT&T SIM card in an iPhone 3GS.

Plug the little SIM card from your carrier into a GSM-based phone (the SIM usually sits in a slot near the battery), and it’ll ring when someone dials your mobile number; remove it and put it into another phone, and the second handset will essentially become “your” phone.

CDMA phones, on the other hand, don’t use SIM cards. Instead, CDMA carriers (like Sprint or Verizon) must program a specific handset to work with your mobile number.

There are a series of other, often highly technical difference between CDMA and GSM (one being that CDMA phones only support a maximum of three people on a call at once, versus up to eight on a GSM phone), but here’s the bottom line: a CDMA phone won’t work on a GSM network, and GSM phones won’t work on CDMA networks.

And unfortunately, there’s one more twist.

Theoretically, you should be able to plug your AT&T SIM card into a T-Mobile phone and start making calls; that said, most carriers “lock” their phones to their respective networks.

For example, the AT&T iPhone is locked to AT&T’s network, meaning you won’t be able to use it on another GSM carrier, like T-Mobile.

Those who want an iPhone that will work on any GSM carrier will have to shell out more than $650 for an “unlocked” version of the iPhone (which is sold directly by Apple, and doesn’t come with any carrier discounts—hence the sky-high price tag).

That’s the bad news; the good news is that some GSM carriers, including T-Mobile, will unlock a phone after a certain amount of time (typically four months or so or so) has passed on your contract; all you have to do is call and ask. (Sorry, AT&T iPhone users: AT&T won’t unlock your iPhone for you, although you can always try “jailbreaking” your iPhone if you’re feeling brave.)

Have more questions about the difference between CDMA and GSM phones and networks? Let me know!

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