Problem after switching Network provider

I had the same problem after switching to Lebara from Plusnet. Tried various fixes but the one which worked for me was as follows.

Go to Settings>Phone>My Number

Edit to the correct number and press Save (ignore that the wrong number is still displayed)

Switch off phone for a few minutes and then restart

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** You may not have an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.

Turn your phone number on or off for your Mac

For Messages, choose Messages > Settings, then click iMessage. For FaceTime, choose FaceTime > Preferences, then click General. Turn your phone number on or off: To turn on your phone number, select it and your Apple ID in the list.

Add or remove your phone number in Messages or FaceTime

Thanks, this worked