iphone calls go straight to voicemail

I was having that problem but for me it was my fault. I had downloaded the AT&T security app and set up some call rules. I later deleted the app thinking that once deleted anything I set up on the app would stop, but AT&T explained that is not the case. Even if the app is deleted the calls are handled according to the rules I set up (for example, silence unknown callers). So, I downloaded the app again and reset/removed the call rules I had established and then deleted the app again.

You have stated that SOME incoming calls go straight to voicemail. If none of the recommendations that are shown below work for you, then be sure to contact your carrier as well to ensure that they have setup your account correctly.

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1. Check to see that Silence Unknown Callers is set to OFF. Open Settings ➔ Phone ➔ Silence Unknown Callers : Set this to OFF
2. 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.
3. 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.