Not all smartphone data plans are created equal, particularly when it comes to overage fees. Some carriers will start charging you by the megabyte if you creep over your monthly data limit, while others may slow down, or “throttle,” you data speeds if you’re too much of a data hog.

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.

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

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.”

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!