'''Try these steps:'''
* Turning off Wi-Fi calling and Voice LTE may fix the problem
* Open Settings and tap '''Cellular -> Wi-Fi Calling'''. Turn off the switch next to '''Wi-Fi Calling on This iPhone''' to turn off Wi-Fi Calling and go to '''Settings -> Cellular''' and tap '''Cellular Data Options'''. Tap '''Enable LTE -> Data Only''' to turn off Voice LTE. You’ll know Voice LTE is off when the blue checkmark appears next to '''Data Only'''.
* Turn Airplane Mode Off And Back On
* Turn Cellular Data Off And Back On
* Check For A Carrier Settings Update open '''Settings''' and tap '''About''' to check for a carrier settings update. If an update is available, a pop-up will appear within about ten seconds.
* Eject your Sim Card and Turn off Your Phone Once the device has been powered down reinsert the SIM and then power the device back up.
* Reset Your iPhone’s Network Settings
* Put Your iPhone In DFU Mode