‘Unavailable for Sharing’ message under Sign-In and Security section on my iPhone

Hi,

Under sign-in and security section, under my mobile number it says ‘unavailable for sharing’, what does this mean? When I click on the info button it says: you can allow people to contact you at (my number) in apps and services using your Apple account, play games using game centre, share photos and files, collaborate on documents and more. If you want to remove this number, you can do that in two-factor authentication settings.

My iMessage seems to be working fine and I checked the settings for iMessage and FaceTime and all looks fine. Any help would be appreciated. As I don’t really understand why it says unavailable for sharing.

[Re-Titled by Moderator]