the sound does not always come through when receiving calls or messages. Have to power down and restart 2 - 3 times a day for sound.

I have an iphone 6 S plus (my phone is less than 4 weeks old) using the iOS 9.2 version. My problem is I do not always get any sound alerts for incoming messages or phone calls. It seems like the only way to get them is by powering down and resarting, but Imay have to do this 2 - 3 times a day. Does anyone else have this problem or know how to fix it?