I solved this issue for myself and my wife. We changed our providers recently. For us the issue was caused when we put the new sims in the phones. It should take 24hrs for the numbers to port over but it took much longer. So we had to reinsert our old sims. Within the phones settings the carrier is still using our old number but the phone also registered the new number, so two numbers. So...
go to iMessage and FaceTime in settings
remove all numbers listed if you can
above Messages in settings go to Phone and if your number is incorrect as ours were change it to the correct number.
go to you Apple ID profile at the very top of settings and go into the Name, phone numbers and email settings. Check the number there, if there is no number select the little blue “Edit” and add your number.
go back and turn off iMessage and FaceTime.
turn off phone and remove the sim.
power up the phone, reinsert the sim.
turn on iMessage and FaceTime. Activate your number.
Its seems like a lot and maybe there’s a quicker way but this sorted this expired number issue for us. Best of luck!