Your phone will continue to work on AT&T.
Until recently cell phones had a voice channel and a data channel. The voice channel was GSM (Global Standard for Mobile) and the data channel was 2G, 3G, or LTE (which is synonymous with 4G). With an iPhone 12 or 13 it can also be 5G.
Your phone had a voice channel and also a data channel that could be 3G or LTE depending on the network you were connected to.
Several things have happened recently. First, a couple of years ago carriers offered Voice over LTE, meaning that your calls would be digitized and use the LTE data channel instead of the voice channel. Now, recently, the major carriers completely eliminated their voice channels, and only use Voice over LTE, so essentially they went all digital. Since all calls now use VoLTE and all data LTE there is no longer an option to select it for voice or any other data option.
If you travel to an area that doesn’t support VoLTE the phone will revert back to the old system, so your phone will remain compatible around the world. It is smart enough to choose the right communications protocol.
To further complicate it, your carrier also has Wi-Fi calling, which means that when you are home on your Wi-Fi network (or anywhere that has Wi-Fi) your phone can use the Wi-Fi connection for calls. This happens automatically if you have this feature turned on. It’s better for the carriers, because it off-loads their towers, and better for you, because Wi-Fi calls don’t count against your calling “minutes” if you don’t have unlimited. And when traveling outside of the US any calls over your hotel’s Wi-Fi doesn’t count as roaming calls.
