Phone periodically not ringing or vibrating

My iPhone 5s has started not ringing or vibrating at times. I've made sure the Do Not Disturb setting is off and hard restared the phone. Still expereinces intermittent failure to ring or vibrate, although I do get a missed call message and the voicemail works to receive messages even when it didn't ring. Very frustrating and causing me to miss call. Any thoughts will be greatly appreciated.