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

The rule of thumb is 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 operators that piggyback onto these larger networks usually follow the same rules as their parent network.

Not all cell phone carriers have the 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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The technical differences between these systems aren’t important for our purposes here. (If you’re interested, here’s a good explainer.) CDMA and GSM both allow your phone to transmit voice calls and text messages across the network, 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 carriers build out their LTE data networks in the coming years, all voice calls and text messages will be routed as LTE data, and older radio technologies like CDMA and GSM will become obsolete. This might make it easier to move phones from network to network, since everyone will be using the same LTE specification. But because the carriers use different LTE configurations and frequency bands, it’s not 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.

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 as your 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 have to 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.

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The best way to figure out 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.

Stephen Layton is a staff writer at NerdWallet, a personal finance website. Email: slayton@nerdwallet.com.

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