How do I change my phone number in Mobile Service on iPhone 12?

Hi I have recently changed my phone provider and have ported my old phone number to my iPhone 12 However I remain mystified that in Settings>Mobile Service>My Number, the number that I was originally (before porting my old number) issued with is still showing and despite my best efforts, I cannot change it to my old ported number. It feels as though it should be easy and it is simple enough to delete the number I want to get rid of and enter the number I want. However it will not save it.

The phone is working perfectly well both for calls and messaging with my ported number despite showing the wrong number on the display.

Tge correct number is showing in the App for my new provider

i remain uneasy about the wrong number showing in settings which I can’t change.

help!

[Re-Titled by Moderator]

Original Title: Change my phone number in Mobile Service