Cannot change my number in settings so imessage/Facetime are wrong
I recently ported to O2 from Vodafone and have had a few issues but one is persistent - iMessage and Facetime includes only the temporary number O2 allocated to me which is shown in the ios Phone Settings "My Number". I have tried multiple times to change "My Number" (including after restarts, flight mode, etc.) but it doesn't seem to accept it - it immediately reverts back to the temporary number. This means I cannot use iMessage with anyone who has my number.
Any advice would be very helpful!
iPhone 11