Changing iMessage to use moved number

Just after a little advice as I bought a Lebara sim, moved across my old number but iMessage is using the new sim number. How do I change it?

When you port a number, your iPhone often "sticks" to the temporary number that came with the new SIM card. This is a classic caching issue where Apple’s servers haven't realized the switch is complete.
Here is the most reliable "flush and reset" sequence to get it working.
1. Update your number manually
Sometimes the phone knows the port happened, but the "My Number" field is still stuck.
Go to Settings > Phone > My Number.
If it shows your temporary number, delete it and type in your ported number.
Tap Save and restart your phone.
2. The "De-registration" Cycle
This forces iMessage to look for a new activation signal.
Go to Settings > Messages and toggle iMessage OFF.
Go to Settings > FaceTime and toggle FaceTime OFF.
Restart your iPhone.
Go back to Settings > Messages and toggle iMessage ON.
Check Send & Receive. Your ported number should now appear with a spinning wheel next to it as it re-verifies.
3. Reset Network Settings
If the above doesn't work, your "carrier bundle" might be confused.
Go to Settings > General > Transfer or Reset iPhone > Reset.
Select Reset Network Settings.
Note: This will wipe your saved Wi-Fi passwords, but it won't touch your photos or data. It forces the phone to re-handshake with the cell tower.
4. Check Date & Time
iMessage activation relies on a security "timestamp." If your time is off by even a minute, the Apple server will reject the activation.
Go to Settings > General > Date & Time.
Ensure Set Automatically is toggled ON.
Important: The 24-Hour Rule
Number porting is actually a "split-service" process. While you might be able to make calls, the SMS bridge (which iMessage uses to verify your number behind the scenes) can take up to 24 hours to fully move over.
If you see "Waiting for activation," it’s often best to leave the toggles ON and wait overnight. Repeatedly toggling it on and off can actually cause Apple’s servers to flag you for spam.

Thank you, that sorted it, many thanks