So you have a perfectly serviceable cell phone and you’re ready to move it to a new carrier. After unlockingyour phone from your current carrier’s network, you face one morehurdle: making sure your new carrier accepts it.

The rule of thumbis that AT&T customers should have an easy time moving their phones to T-Mobile and vice versa.Verizon and Sprint customers will have a harder time. Mobile virtual network operatorsthat piggyback onto these larger networks usuallyfollow the same rules as their parent network.

Not all cell phone carriers havethe same ability or inclination to accept new devices onto their networks. In the U.S., this is because of a division in the radio technology that the major carriers use. Verizon and Sprint use code division multiple access technology. AT&T and T-Mobile use Global System for Mobile communication. Most of the rest of the world’s carriers use GSM as well.

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Different and incompatible

The technical differences between these systems aren’t important for our purposes here. (If you’re interested, here’s a good explainer.) CDMA and GSMbothallow your phone to transmit voice calls and text messagesacross thenetwork, but in two different and incompatible ways. Thus, you can’t bring a phone designed to work on Verizon’s CDMA network over to AT&T’s GSM network.

As carriersbuild out their LTE data networks in the coming years, all voice calls and text messages will be routed asLTE data, and older radio technologies like CDMA and GSM will become obsolete. This might make it easier to move phones from network to network, sinceeveryone will be using the same LTE specification. But because the carriers use different LTE configurationsand frequency bands, it’snot guaranteed. (Here’s an in-depth look at LTE.)

For now, U.S. carriers use a combination of LTE with CDMA or GSM to route voice calls, texts and data around their networks.

Is your phone acceptable?

Customers on a GSM network can easily take their phone to another GSM provider. That’s because the GSM specification requires carriers to accept any GSM-compatible phone.All GSM phones have SIM card slots, and as long asyour phone is unlocked,you can just pop in a different network’s SIM card to get service. This also means that GSM phones travel abroad much easier, as most international networks are GSM.

CDMA carriers are pickier about the phones they allow on their network and don’t haveto accept any devices they don’t want to, including phones from other CDMA carriers.So while Verizon and Sprint both use CDMA, they won’t necessarily accept each other’s phones.

» MORE: Best cell phone plans

The best way to figureout if your phone is acceptable is to find the “bring your own device,” or BYOD, page for the carrier you’re moving to and input your phone’s information. Here are the BYOD pages for Sprint and Verizon.

CarrierNetwork Coverage ProviderGSM or CDMA?Bring your own device?BYOD CheckerVerizonCDMANot likely, some CDMA devicesLearn moreAT&TGSMLikely, most GSM devicesLearn moreSprintCDMANot likely, some CDMA devicesLearn moreT-MobileGSMLikely, most GSM devicesLearn moreKing Street WirelessCDMANot offeredN/AAT&TGSMLikely, most GSM devicesLearn moreSprintCDMANot offeredN/ASprintCDMASome Sprint phones onlyLearn moreT-MobileGSMLikely, most GSM devicesLearn moreVerizon, AT&T, T-Mobile and Sprint
Depends on network coverageLikely, some GSM and CDMA phonesLearn moreVerizon, AT&T, T-Mobile and Sprint
Depends on network coverageLikely, some GSM and CDMA phonesLearn moreVerizon, AT&T, T-Mobile and Sprint
Depends on network coverageLikely, some GSM and CDMA phonesLearn more
Sprint, T-Mobile and U.S. CellularDepends on network coverageProject Fi only works with the Nexus 6, 6P, 5X and the Google PixelN/AVerizonCDMANot offeredN/AAT&TGSMLikely, most GSM devicesLearn more Sprint or T-MobileDepends on network coverageUnlikely, only some Android phonesLearn more

Stephen Laytonis a staff writer at NerdWallet, a personal finance website.

The article Why You Can’t Always Bring Your Phone to a New Carrier originally appeared on NerdWallet.

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