How to resolve 'This number is registered to your Apple ID, but is not associated with this phone' issue on iPhone?

I keep getting this message on my iPhone:

This number is registered to your AppleID, but is not associated with this phone. You can keep using the number for iMessage and FaceTime until it expires.

I haven’t changed providers but have 5 days apparently until my number expires?? iMessages I send and receive are now green. I’ve checked all troubleshooting and turned off iMessage and FaceTime, turned them back on again and restarted the phone? Any help would be appreciated. Thanks.

[Re-Titled By Moderator]