Check on unlock status

If T-Mobile requests an unlock Apple will unlock it. There's no discretion involved on Apple's part. However, not all T-Mobile phone reps have the same level of competence, so call again and ask someone else. Note that this is true of all carriers, I'm not particularly singling out T-Mobile.

One way to check is to restore the phone to factory settings using iTunes. If it is unlocked you will see this in iTunes:

If you’re asking about unlocking from a cellular carrier, then Apple has no involvement at all in that process at any time.

Not all iPhones support all carriers - in fact, almost all do not.

So if the former carrier says the iPhone is unlocked from their perspective, you may well be trying to move to a carrier that cannot be supported with this particular iPhone model.

You might check Settings > General > About > Model, and report the result here, along with the previous and new cellular carrier names.

The good news is that model is an iPhone 7 which can be used on both T-Mobile and AT&T.

Might there still be some financing going on through Apple? Is the iPhone not fully paid for yet? Anyway, the final unlock decision will be T-Mobile’s.