Is buying an AT&T iPhone 7 Plus outright in full from local Best Buy mean it will work on T-Mobile too?

Looking to find out if buying an att iphone outright from my local Best Buy is going to work on T-Mobile too? I don't believe ATT would lock a phone to their network prior to it even being activated by a customer on their network. But I have T-Mobile and Best Buy doesn't sell T-Mobile phones so trying to make sure this will work.

Only of the phone is UNLOCK from AT&T, you will need to ask that directly up front before you hand your money over.

And the question is, "Is this phone unlocked, so that I can put any GSM SIM card in it so that it will work on that carrier?"

Best Buy would call it an unlocked phone if it was.

If you are buying it outright & want to guarantee it would work on T-Mobile may as well just buy one from Apple, SIM free, & then put in your T-Mobile SIM.

You could enquire for sure if it is unlocked but it is likely going to be locked. The phones are usually locked before activation if sold in that configuration. Of course if unlocked then they are so prior to activation.