I encountered the same after resetting settings, having problems with Files app. The following steps helped me to get phone number back:
1. Go to Settings->Your name (Apple ID, iCloud, etc)->Name, phone numbers, e-mail
2. Press Edit over Contact details section
3. Press Add e-mail or phone at the bottom of the section
4. Choose Add phone number
5. Add your phone and press Next (it might require SMS confirmation, I don't remember)
6. Your phone number should appear in the Contact details section now
7. Choose added phone number in Facetime and Message settings. It should be available there now. But Settings -> phone is still Unknown.
8. Restart iPhone. Settings->phone should show your phone number.