Sometimes, when you don’t pay too much attention, the reason why your phone is not ringing may be the fact the Mute is on. To check this, look at the Mute switch which is located on the side of your device. The mute is turned on when the orange line is visible inside the switch.
Navigate to Settings, and click on Do not disturb. If the Manual is on, turn it off. If the Scheduled is on, check the times in which this option is set to be used. Maybe the phone did not ring because it was set that way.
Go to Settings, tap on General, tap on Reset, and click on Reset network settings.
From the Home screen, click on Settings, and choose Cellular. Click on the Enable LTE switch. Turn off 4G.
This solution may help, but make sure to back up your data first. Click on Settings, click on General, choose Reset, and find Erase all Content and Settings. Click on it. You will see the message in which you can choose to go on by tapping on Erase iPhone or to cancel. Choose option No.1. This usually lasts for several minutes, sometimes even an hour. When it is all done, set up your device as new.