Incorrect number associated with iPhone after porting my number to a new SIM

I have an iPhone 13 Pro running iOS 18.3.2

A couple of weeks ago I got a new SIM with a new provider, and had a new number for around a day until they finished porting my old number across with a PAC from my old network.

If I dial *#100# I can see the correct number pop up on screen, though I assume this is a network-specific tool.

The temporary number is now associated with my iPhone when I look at my phone in Settings->Apple Account->iPhone 13 Pro. I've deactivated the temporary number, and re-added my old number to my account, but I'm seeing a message that it's "Unavailable for iMessage and FaceTime".

I've tried restarting, removing and reinserting the SIM, disabling & re-enabling Messages and FaceTime, and resetting my Network Settings, but the old number is still there.

Are there any other things I can try to remove the number that I don't have access to and get my iPhone working properly again?