Coming back to Xfinity mobile

Originally bought phone IPhone X from Xfinity then left and used phone on 2 other carriers. Now Xfinity says the iMei is not compatible. But is same phone unlocked and worked on Xfinity 3 months ago. I was able to locate the original iMei number for the phone and it processed the order to get a new sim shipped but the phone doesn’t have the original iMei since it changed carriers. I’m assuming I can still add this phone when the new sim arrives (lost the old sim) but what steps need to be taken to port over and keep my number still?