iPhone doesn't ring for incoming calls.

1. Check to see that the Silent switch is not set to ON. Silent switch is located on the Top Left Sideof your iPhone.
2. Open Settings ➔ Sounds & Haptics ➔ Ringer and Alerts : Make sure this is not set to OFF or too Low. Set Change with Buttons to OFF. Below that Make sure that there is a Ringtone selected.
3. Check to see that Silence Unknown Callers is set to OFF. Open Settings ➔ Phone ➔ Silence Unknown Callers : Set this to OFF
4. Make sure that a ringer sound is assigned. Open Settings ➔ Sounds & Haptics ➔ Ringtone : Ensure that a Ringtone is assigned here.
5. Check to see that Do Not Disturb is set to OFF. SWIPE DOWN from the TOP RIGHT CORNER of your screen. This will reveal the CONTROL CENTER. Find the FOCUS button (should be highlighted in white if Do Not Disturb is ON) and Tap on it. This should turn OFF Do Not Disturb.
6. If all of the above is OK on your iPhone then Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.

Could this be, if you looking at the phone while the call comes in, it will not ring or is muted low.

Try calling your phone from another phone and make sure not to look at screen. Does it ring now ?