IPhone prompts how to set "Mobile phone number is not associated with this phone"

Some users will see this prompt in the settings when using iPhone: This number has been used to register your AppleID, but it is not associated with this phone.

This prompt may be related to a trusted phone number. You can check the settings first.

View trusted phone numbers:

Open iPhone Settings -AppleID- Passwords and Security, and view trusted phone numbers here.

If the current trusted number is not a phone number bound to an AppleID account, you can gently edit it and then click Add Trusted Phone Number to add it.

It is suggested that you can fill in a phone number that can be used normally by signal, so that when you need to log in to other devices through AppleID and change the ID password, you can receive SMS for verification.

If you have filled in the trusted phone number, the above prompt will still appear, and you can continue to try:

Reseat the SIM card; Turn off iMessage in Settings-Messages, then turn it back on and activate iMessage with this mobile phone number.