Carrier T-mobile SIM in AT&T iPhone 7

[TypeMRT]

I bought an AT&T iPhone 7. I want to know what will happen if I take out the AT&T SIM that comes with it and put in a T-Mobile SIM instead and then do the activation. Does it come pre-locked to the AT&T network or does it come unlocked until its powered-up with the AT&T SIM for the first time?
Thanks.

 

[winterny]

If you bought it from apple.com or the apple store, and you paid in full (or financed from apple), it will be unlocked and will work with your tmobile sim.

If you bought it from AT&T, or if you bought it from the apple store with AT&T financing, it will be locked to AT&T and won't work.