This worked for me yesterday
Since I have AT&T I signed onto my ATT App and located the send a refresh to my device Do not reboot your device when it ask you to -
Then I went into my settings - General - Shutdown - Transfer or Reset iPhone - Click Reset - Now select Reset Network Settings - now when prompted to restart the iPhone do so
Give it a few minutes then turn your iPhone back on you "should" see your carrier or in this case AT&T reappear now try making a call on carrier network if it works you are good still but give it a few minutes as mine kept going into SOS and back out but eventually it stuck on the carrier network
Then simply reconnect to your WiFi network - working