Off-contract phones are great, particularly if you want to save some money on your contract, or go prepaid. Fortunately, it's simpler than you think to know which devices are compatible with your network.

If you buy your phone from the carrier itself, you probably won't need to worry about which network your phone runs on unless you plan to travel internationally (in which case, feel free to skip to the last section). However, if you're planning to buy a phone off-contract and bring it to your carrier, there are some compatibility questions to ask first.

Is Your Phone CDMA or GSM?

There are two main standards for cell network towers used worldwide: GSM and CDMA. GSM is used by carriers like AT&T and T-Mobile. Globally, GSM is a much more popular standard and many carriers utilize this network type. CDMA, on the other hand, is used by Verizon and Sprint and while it is used in some countries, it's not as universally common.

One of the key differences between the two is that GSM phones use removeable SIM cards while CDMA phones don't.With GSM phones, you can pull out the SIM card and place it in a new device and continue using your service uninterrupted. Some newer CDMA devices with LTE use a SIM card for 4G service, but the device is still tied directly to the network and you'll need to get your device activated with the carrier before you can use it.

Once you've figured out whether your carrier is CDMA or GSM, you're inside the ballpark. However, if you plan on using 3G, LTE or another type of 4G network (and of course you are), there's a bit more info you need to confirm.
What 3G/4G Bands Does Your Handset Support?

If you're using a device with a 3G or 4G radio, you need to make sure that your device supports the proper bands. Many modern, high-end phones come with multi-band support so they can run on a couple of different carriers, but it's especially important to check. If you buy, say, a Nexus phone unlocked from Google that specifically states it supports your carrier, then you're fine. However, if you're buying an unlocked international device, you need to check the bands.

Domestically, here are the common bands that each carrier supports:

AT&T: 700/1700/2100
Verizon: 700
T-Mobile: 1700/2100
Sprint: 800/1900/2500
MetroPCS: 1700/2100/2500

Of course, just matching up one of the numbers isn't necessarily enough. T-Mobile, for example, uses both 1700 and 2100 bands, but may not use them simultaneously in all areas of the country. If your phone supports the 1700 band, but you're in an area with coverage in the 2100 spectrum, you'd be out of luck. If you need to clarify this issue, call your carrier and ask them if your phone is supported.
Do You Need to Travel Internationally?

Most of the information above is overkill if you're buying an off-contract phone sold in the US and don't plan to leave the country. It gets a lot more important if you ever intend to travel. Carriers like Verizon will market "global ready" phones that operate on CDMA networks at home but still include GSM radios for traveling. Meanwhile, regular GSM phones that include multi-band radios support a wide range of frequencies.

While most countries use GSM networks, band usage can vary from carrier to carrier. When you're choosing your device, it's important to know what carriers you might need to use your phone on and check with them before buying. There are online tools available that can help you find out if your phone is supported in a particular country. Most major domestic carriers also offer plans that support using their own or partner networks overseas. However, it may be cheaper to just swap out a prepaid SIM card while you're away, so it's worth shopping around.

Ultimately, while a lot of this information will help you narrow down your choices, the best way to confirm that a phone you want works is to call the carrier you want to use it on and ask. You'll spend less time on the phone if you're prepared and do your research beforehand, but always ask.