Ported Phone Number Conflicting with SIM Number - Can't Receive Phone Calls

I have changed network provider from 3 to O2 - my number has ported from 3 to O2 (so the 'phone number originally associated with the SIM no longer receives calls, as it obviously doesn't 'exist' anymore), but I cannot receive calls on my original number (it just goes to voicemail).

I can make calls, and it appears to come from my original number (which is what I want).

I think I have found the problem - the 'Phone' section of settings lists 'My Number' as the number that was originally allocated to the SIM, but I cannot change it. I have tried to change it, but it doesn't save it.

My original number is listed as a number I can be reached by under 'FaceTime' and 'iMessage', but it says it is registered to my Apple ID but is not associated with the phone, and it will expire in 29 days.

Has anybody got any idea how I rectify all of this? I am currently unable to receive calls at all (other than through WhatsApp, which is fine with the original number).

Thanks in advance.