I have a message that reads This number is registered to your Apple ID but is not associated with this phone. You can keep using the number for IMessage and FaceTime until it expires in 13 days.

Hey there 17or25,

Thanks for using Apple Support Communities. If we understand correctly, you're having an issue with your telephone number and iMessage/FaceTime. Is that correct?

We see you referenced Add or remove your phone number in Messages or FaceTime

That is a good resource.

Is the message correct in stating that the telephone number is not associated with your iPhone? You can see the numbers associated with your Apple ID account at Settings > Messages > Send & Receive.

If it is not, you should associate the telephone number if you wish to keep it. If it is, we recommend contacting one of our Advisors here for further assistance: Contact Apple for help with Apple ID account security

Kind regards.