For locked AT&T phones you’ll want to visit this web form. Full documentation is available here.

For Verizon devices you’ll want to call 1-800-711-8300 and ask for a SIM unlock. Full documentation is available here.

Sprint phones can be unlocked by calling 1-888-211-4727. You can also request an unlock through a web chat. Full documentation is available here.

T-Mobile customers can unlock their devices by calling 1-877-746-0909. You can also request an unlock through a web chat. Full documentation here.

Here’s more information on how to unlock your phone on the big four U.S. carriers, including how to verify that your device is eligible, caveats in the fine print, and alternative carriers you can bring your device to. The FCC also has useful information about the process.