iPhone 'My Number' is missing in the settings app - can I get it reinstated?
I can’t reproduce this, but here is how I would troubleshoot it and why:


Put the device in AirPlane mode and then power off.
Start up and then go to settings (still in AirPlane mode) and look at the cellular data information / carrier information / phone information. Perhaps record/screen shot the MEID/IMEI and SIM information.
Open and re-seat your SIM card while in AirPlane mode still.
Connect to WiFi and check for updates, update the iOS to the latest and then disable AirPlane mode.
Power down and back up one more time while cellular and WiFi are enabled.
In the settings app - reset network settings - this should restart one last time.


At this point, you can be sure the SIM isn’t trivially failing and the iOS had a chance to talk to the cellular carriers. At this point, I would back up the device and then erase all content and settings. Now make a tea or coffee and get ready to call/engage tech support for your cellular carrier and explain you’re needing help knowing why your device isn’t working with their service. They can’t really say there’s a bug with iOS or your apps if you’re willing to repeat the reset network settings / power cycle / erase all content and settings. They might say - check with Apple and Apple would say, restore the device and if it still doesn’t work go back to the carrier. 

At that point, the carrier is really on the hook for providing you a reason why your service isn’t working. Similarly, Apple support will be a breeze to erase / restore / rule out trivial failures and fix this in almost all cases without waiting hours on either call.