Q: iMessage only shows old number - how do I get it to show my new number?

I changed phones and mobile number at the same time, inserted my new SIM card in the new phone and then restored it with the backup of my old phone. Now, whenever I try to turn on iMessages, my old number is still displayed (and greyed out), and iMessages are not turned on for my new number.

I can send iMessages, but only from my email address.

How do I get iMessages to fetch my new number from the SIM card?

Here's what I've already tried:

- Turning off both iMessage and FaceTime, entering flight mode, turning iMessage back on.

- Turning off both iMessage and FaceTime, turning off my phone and leaving it off for 10 minutes before turning it on again. Then turning on iMessage.

- I have checked that my phone number is entered under "My number" in contacts and that my date and time settings are correct.

- De-registering my old phone number from iMessages using Apple's online tool.

Nothing so far has worked.

Please help.

Thanks