The reason this happens is that your apple ID has the temporary number registered, so to resolve this sign out of your Apple ID, turn phone off and then back on, login back to your Apple Id, this should resolve the problem.
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I have the same problem and just attempted the suggested solution and it hasn't worked. My Apple ID is still showing my new and old device with the new device tagged with a new number but only for iMessage and Facetime?? Tried to every way to speak/text virgin but they seem closed other than for new deals.