iPhone 11: This Phone Number Cannot Be Used to Sign in

I have transferred my mobile service from an iPhone 7 to an iPhone 11.

The iPhone 11 is working (voice/text/apps) and is paired with my iPad for iMessage and Facetime.

The iPhone 7 had been paired with the iPad for iMessage and FaceTime but is no longer associated with my Apple ID (it was deactivated and erased as part of the transfer to the iPhone 11).

My email address and mobile number are shown correctly In Apple ID - Name, Phone Numbers, Email.

However, under my mobile number is the message "Unavailable for sign-in" with an information symbol next to it. The same message appears in my iPad. When touched, the information pop-up is:

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This Phone Number Cannot Be Used to Sign In

(mobile number) is being used for iMessage and FaceTime. It is unavailable for sign-in because it's already in use with another account.

If you want to remove this number, turn off iMessage and FaceTime on it first.

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Going to my Apple ID account, it shows my mobile number as trusted. I do not believe that my mobile number has ever been associated with another Apple account (I have had it since before there were iPhones).

Disabling iMessage and FaceTime on my iPad and restarting the iPhone 11 had no effect.

The iPad is paired again with iMessage and FaceTime on the iPhone 11.