I use an iPhone 12 Pro Max and I also have an iPhone XS Max and an iPhone 6 connected to my Apple ID. However, the phone number in my 12 Pro Max is not showing in the FaceTime and iMessage settings and it says that activation is unsuccessful. I would highly appreciated any help with regard to this situation. Thank you!
See if anything here helps.
Messages or FaceTime - Add or remove your phone number.
See the Apple Support article(s) below.
Messages - If you get an error when trying to activate iMessage or FaceTime. *
If you can’t turn on or sign in to iMessage or FaceTime. *
Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch *
Try a forced restart.
Reset Device. -
Hard reset. -
Do a backup and try a factory reset. Then restore from your backup.
Restore to Factory setting. *
Restore - If you see an error when you update or restore your iPhone, iPad, or iPod. *
Backup - Restore your device from an iCloud or iTunes backup. *