Many iPhone 6/7/8 and iPhone X user are facing issue with iMessage and Facetime that they can't see their phone number in iMessage and face time but it only shows apple id.
The problem is reported by many users and even after trying with tech support they are able to fix the issue. Even after resetting iMessage and Facetime settings did not help to show correct phone number in place of Apple id.
This can happen if your phone number is not correctly associated with Apple id.You need to get your phone number associated with your Apple ID, than it should show phone number in iMessage.
Here in below link you have some general steps that are released by Apple to fix the issue, but many users complains that this does not help them to fix the issue:
In case this does not work for you Please try to fix iMessage not showing apple id instead phone number:
First go and check if your Apple ID account is set up with your right phone number.
Now go into your account and see under “Reachable at” that your phone number is listed. If not click on edit and add your number.