5s calls go directly to voicemail--no ring

I have an iphone 5s. verizon is my carrier.

Sometimes a call goes directly into my voicemail without ringing.

It is not constant. I may get one phone call and then 10 minutes later (and i am in the same place) I see a message that someone called, but the phone did not ring. 30 minutes later, a call comes thru.

this has happened in different locations.

i don't have do not disturb on, nor the mute button.

any solutions?