I just had this exact notification that would not go away and I tried absolutely everything, but this is what worked for me!
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Go to Settings --> Messages --> Send & Receive
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You should see the same notification about the phone number expiring in the "Send & Receive" tab again, but this time with a red "Remove From iMessage and Facetime" button underneath. Press this button.
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Now under Send & Receive you should see your phone number still there, but without the checkmark next to it that indicated that it is active.
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Simply press on your phone number and it will activate it again with both iMessage and Facetime again. You should see that small checkmark next to it again.
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This is what took the notification away for me, no messages or anything were lost in the process by pressing remove.