iMessage / FaceTime incorrect number

Hey all - I swapped phone numbers from my old SIM to a new carrier, but the iMessage and FaceTime numbers registered remain that of my old number.

The old and incorrect number is ‘greyed out’ and cannot he changed.

how can I change the old number to my existing one under the ‘you can be reached at’ section? People who try to contact me via FaceTime / iMessage cannot do so as it is contacting an old device that I am not using (calls / messages come through on my old phone with no SIM via WiFi).

many thanks.

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