Strange I did not see the Mobil no is the iMessage setting? All search on Google was more confusing ….
Found the problem: if the phone is setup without a SIM (Typical in India: the Nano SIM is not Available most of the time), the Mobil No is not picked up in Setting > Phone > My Number
The solution is
1. Enter your Mobil no at Setting > Phone > My Number
2. Switch Off the iMessage Setting > Message > iMessage
3. Switch off the iPhone
4. Switch On the iPhone
5. Switch ON the iMessage Setting > Message > iMessage
6. Confirm ok at the pop up message.
That’s it you are good to go.