old number hasnt switched and cant send sms texts

Hi there @Joseph Ibbs-George, welcome to Community!

Please try the following depending on which phone you have:

For iPhone:

1) Turn off iMessage (Settings > Messages)
2) Restart your phone (by pressing and holding the on/off button)
3) Tap Settings > Phone > My Number and confirm that it is correct. 
4) If it is not correct, please enter the correct number
5) Turn iMessage back to On (Settings > Messages)
6) Finally, go to Settings > Messages > Send & Receive and see if the correct number is displayed under "Start new conversations from:"

If you are having issues with step 4 please reset network settings and try again.

Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings.

For Android:

1) Open Messages, go to Settings (top right, Message Settings), open "Chat Features" or "RCS Chats", verify if this is the correct number or not.

2) If it shows the incorrect one, then head to: https://messages.google.com/disable-chat and you should be able to unregister the number. Once done, turn off the Chat Features/RCS and the phone. Wait a few minutes and turn the device back on and re-enable Chat Features/RCS it should let you re-register/update to the correct number.

Alternatively, you can turn Chat Features/RCS off and test SMS after this.

Thanks,

Tyler