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?