My iPhone 7 was repaired and issued a new serial number. Now the carrier says the phone is not compatible with its services. The carrier seems to consider the phone new based on the MEID number, however. They never asked for any serial number

What is an "iPhone 7 2"? The carrier referred to the phone as a 'new phone' throughout the conversation even though I said it had simply come back from repair with a new serial number. Or is Apple actually issuing a new phone and not being upfront about it? I understand why Apple might not ask customers who their carrier is, but in this case it's invalidated their repair.

From the phone itself I merely get the repeated notification that the SIM card is locked and I must contact my carrier. The SIM card is the same, Apple gave me my card to take home with me when I left the phone with them for the repair.