Why do I have to turn on and off my I Phone (ois 9.1) to get a ring tone after using safari and mail and stocks? Mute , do not disturb are not on. The volume is turned way up.

I have an I Phone 6 with IOS 9.1. I have experienced this problem since the updates. When I use stocks, mail, or safari, and close out the page, my phone will not ring UNLESS I turn it off and on after using the aforementioned programs. Volume is up, no mute, no do not disturb. It's as though these programs cause an interference to the sound. I can see the phone ringing, answer it and no sound. If I immediately call that number back, I have sound as well. Solutions?