The mobile number i used and saved in my authentication is no longer in service what can i do?
iPhone 7
See the Apple Support article(s) below.
Passcode - If you forgot the passcode for your iPhone or your device is disabled. *
https://www.youtube.com/watch?v=7s8-zLG9ySQ
You need to change/add the number on your Apple ID. Look in the Security section when signed in online. You may have to add the new number before you can remove the old number. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.