my iphone 5 has started in the last 6 months(phone was purchased in December 2012)frequently not ringing or vibrating with incoming calls. not always--sometimes it rings--but more and more often. if the caller leaves a voicemail, it sounds. This is getting to be a problem. where I am does not seem to matter. right now the reception shows as 4 bars and I just saw a call had come through and not rung. help.