Not sure how other users who answer Apple related technical questions can help with this one. Only the carrier can unlock the phone and Apple is not a carrier. Apple is not involved in either locking or unlocking a phone.
I spoke with an Xfinity support person who says my phone is unlocked but nothing has changed.
The support person at Xfinity is obviously not telling you the truth. All that you can likely do at this point is ask to speak to a manager and patiently explain things all over again.
It might help if Xfinity has a store location where you can take the phone and demonstrate that the phone is locked, even though you have been told that it is not.
When the phone is unlocked, everyone will be able to see this on the phone.
Good luck.