Associating phone number to iPhone after switching networks

Why is it so complicated to keep your existing phone number for iMessages/FaceTime when switching networks? My wife has an iPhone and a Wi-Fi Watch and has recently switched network providers. She put in the new sim card which obviously comes with a different phone number, then entered the PAC code and her old number ported to the new SIM card the following day and is working fine for phone calls/texts etc. Yet under Settings/Phone the number is still showing as the temporary one, and although you can enter the correct one it doesn’t save the change. On both iMessage and FaceTime she has the message ‘ 27 days left’ This number is registered to your Apple ID but is not associated with this phone. You can keep using this number until it expires’. So what exactly does she have to do and why does Apple not simply walk you through the process - I have looked for hours and cannot find a simply explanation. She has tried switching off Message & FaceTime and switching phone off and on but that didn’t work. Do you have to log out of your Apple ID - but then you get a warnings saying car keys will be permanently deleted and will need to reset up - we use Car Key for Tesla. Do you have log out of Watch as well? Surely this is something people do frequently - why is so difficult to understand?