From ATT?
It’s an unlocked phone direct from Microsoft. You could use it on AT&T or T-mobile in the USA or probably any of the smaller carriers who use GSM.
I’ve heard rumors that Microsoft was going to release a version compatible with Verizon bands but I’m not sure if that will happen.
Personally, I think they should have released it supporting every band, much like Apple does with their iPhone and let people decide. Of course it’s still up to the network provider to permit the phone to connect.