sms delivery failure

I had the same problem on a new iPhone 13 which I bought in late April 2023 on ID mobile, when I ported my old long-term number from BT Mobile about 2 weeks later.  Fixed it today 😀 .  

I tried all the settings changes I could find.  I tried Tom’s suggestion above (“Go to Settings > Phone > My Number.  Replace the temporary number you find there with your own number. use +44 instead of the first 0.  Then tap Save. “).  But the iPhone won’t allow it.  After Save, it just ignores the change and goes back to the temporary SIM number.

The answer for me was a Network Reset.  Scary thing to do but it was fine.  Just make sure you pick the right Reset type.  It seems to rebuild all the phone number settings correctly using the ported number.  

To do it, go to: Settings - General - Transfer or Reset Phone.  Tap Reset (near the bottom of the screen).  It gives you 5 reset options: choose Reset Network Settings.

Afterwards you will have to login again to your WiFi networks, using the WiFi password.  You may also need to check your phone, message and iMessage settings.  Mine kept all the Bluetooth connections.

This seems to be a common problem for both iPhones & Androids after porting a mobile number.  I hope this fix works for you.