Can I use another carrier with my AT&T phone?

Episode 1417 (1:17:40)

Tom from Chicago, IL
T-Mobile SIM

Tom traveled to see the eclipse and says that there's nothing that can prepare you for seeing totality. It was amazing. He didn't want to waste the moment trying to take pictures, but he did get one with his iPhone.

When he was down there, he had no cell service. Someone said he could put a T-Mobile SIM card in it and use it. Leo says he'd have to be sure his phone is unlocked to do that. If it's paid off, or if he's in good standing and traveling overseas, he can ask his carrier and they usually will unlock it. He'll also want to be sure it can support the radio frequencies of the other carrier.

Image Taken by User:Omegatron using a Canon Powershot SD110 (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons