Is the unlocked iPhone 4S compatible with my carrier? (reader mail)Linda writes: I was wanting to buy the unlocked version of the iPhone 4S for my son for Christmas but wasn’t sure if the phone was compatible with the carrier in our area. Help!

Hi Linda! So, here’s the thing with the unlocked iPhone: it will only work with a carrier that operates a GSM (or “Global System for Mobile Communications”) network, and not on carriers with CDMA (“code division multiple access”) networks.

Here in the United States, for example, the unlocked iPhone 4S will work on AT&T or T-Mobile, which are both GSM carriers, but not Sprint or Verizon Wireless, which are CDMA carriers.

Related: Why AT&T and T-Mobile phones won’t work on Sprint or Verizon (in the know)

Is the unlocked iPhone 4S compatible with my carrier? (reader mail)

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

How can you tell whether your carrier is GSM or CDMA? Simple. A GSM carrier uses something called a SIM (short for “Subscriber Identification Module”) card to identify a specific phone on its network. These little plastic chips can be swapped in and out of a GSM-based phone, allowing it to connect to the particular network to which a given SIM card is tied.

So, if you’re curious whether your carrier runs a GSM network, just check your cell phone for a SIM card slot; it might be on the side of the phone (as in the case of the iPhone), or inside the handset itself, usually behind the removable back panel and near the battery.

Can’t find a SIM card? Then you might have a CDMA-based phone, which doesn’t use a SIM card—and that means your carrier runs a CDMA network. (If in doubt, you could always just call your carrier and ask, of course.)

Side note: The new iPhone is, in fact, a so-called “world phone” that will connect to both GSM and CDMA networks. That said, Apple has specified that the unlocked iPhone 4S will only work on GSM networks.

If you do decide to get an unlocked iPhone 4S for your son (lucky him!), you’ll need a special type a SIM card from your local carrier: a “micro-SIM,” which is basically a smaller version of a SIM card. Be sure to make the distinction when placing your order, as a regular SIM card won’t fit in the iPhone 4S.

Still have questions? Let me know!