Unlocked

T-Mobile lied to you which they are notorious for doing. Apple cannot unlock the phone, only the carrier it is locked to can unlock it.

Call T-Mobile back and escalate the call to a higher tech to see if they will unlock it, but don't expect miracles. You may be stuck with a T-Mobile phone.

It is very simple.

Only the carrier can unlock the phone.

Apple is not a carrier.

The carrier is lying to you. T-Mobile will probably not unlock the phone. Try to sell the phone to another T-Mobile customer.