Calls go directly to voicemail

Had the same issue a few months ago. After lots of research, still do know why it happened. Here’s a hack to get it back:

go into settings>phone>silence unknown calls. Turn that on for 20 seconds and at the same time turn on do not disturb and airplane mode. Then turn off airplane mode, do not disturb, and silence unknown calls

You have not revealed which iPhone you are using and also which iOS version is installed on your iPhone. It would have been much easier to answer your question had you provided that information.

1. Check to see that the Silent switch is not set to ON. Silent switch is located on the Top Left Side of 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. Open Settings ➔ Focus ➔ Do Not Disturb : Set this to OFF
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.