There’s a bug in iOS 12 Beta 2 which might delete your number from iMessage after installing the Beta 2 update on your iPhone.
Multiple users over at Reddit have confirmed this problem, and thankfully, there’s an easy fix as well for the issue.
- Go to Settings » Messages.
- Turn off iMessage on the device.
- Turn on Airplane Mode.
- Power Off your iPhone and then turn it back On.
- Go to Settings » Messages, and turn on iMessage.