Try these steps: (1) Turning off Wi-Fi calling and Voice LTE may fix the problem (2) 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. (3) Turn Airplane Mode Off And Back On (4) Turn Cellular Data Off And Back On (5) 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. (6) 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. (7) Reset Your iPhone’s Network Settings (8) Put Your iPhone In DFU Mode