Hello Duncaninlondon,
Welcome to Apple Support Communities. It looks like you're having issues with iMessage since you switched carriers, and phone numbers. We'd be happy to look into this with you.
Have you tried going to: Settings > Phone > My number and checking the correct number is there? If not, try to edit it, and enter the correct number.
If the correct phone number is showing there, turned off iMessage as well as FaceTime, restart your iPhone normally, then turn on both iMessage and Facetime.
Please check out this helpful link:
Add or remove your phone number in Messages or FaceTime
Thank you for using Apple Support Communities. Have a nice day.