New phone
How do u sign back into iMessage after it signs you out?
iPhone 11, iOS 15
Question marked as
To activate iMessage and FaceTime using your Phone number
First, check to know whether you have an SMS plan enabled at least for One SMS with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you
If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS Only)
If you can't turn on or sign in to iMessage or FaceTime - Apple ...
If the above process does not work then simply disable WiFi and enable iMessage over 4G