No Sound for Incoming Calls

I have an iPhone X, there is no sound for incoming calls, i.e. the phone does not ring at all when someone is calling me. The phone vibrates (because I have Vibration turned on and there is a banner, just no sound. The audio for the phone is otherwise fine. I have checked the volume and the notifications in Settings and I cannot see why there would be no sound. I have tried turning the phone off and on. I have checked with my service provider (Giffgaff/O2) and they tell me it must be a hardware problem.