my phone is not ringing calls go to voice mail

What model phone?
What software?
What carrier?

You can control what is happening with the ringing tones if you want.
Install this macro controller
* MacroDroid - Device Automation - Android Apps on Google Play

and use it to set the volume levels when the screen locks, and then again when the screen is unlocked.
That is how I certify that my Ringer Volume is always what I want.

 

I am also having a similar problem. it goes away after rebooting but comes back later. I see this thread went dead. Was you issue resolved? How?