Watch this GIF to know how to add the Phone Number
Set up your phone number
On your iPhone:
- Go to Settings > Messages and make sure that iMessage is on. You might need to wait a moment for it to activate.
- Tap Send & Receive. If you see “Use your Apple ID for iMessage,” tap it and sign in with the same Apple ID that you use on your Mac, iPad, and iPod touch.
- In the "You can receive iMessages" section, make sure that your phone number and Apple ID are selected.
- Go to Settings > FaceTime and make sure that your phone number and Apple ID are selected in the "You can be reached at" section.
On your other devices, you can now turn on the phone number that you just set up:
If you use this feature and share your Apple ID with someone, you can't prevent them from seeing your conversations. Instead of using the same Apple ID as someone else, set up Family Sharing.
Add or remove your phone number in Messages or FaceTime - Apple ...
https://support.apple.com/en-in/HT201349