Hi @Mc22 

We’d recommend the following;

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 rDevice] > Reset > Reset Network Settings.
 

Tom