Ihad the same problem. They added in a new feature and it is turned on. Power phone off, power back on. On the bottom of the list it says auto reply that is checked. Uncheck it to turn off.
I have a Kyocera Hydro that won't ring too. Apparently this started happening right after they did a mandatory system update through Boost. I haven't been able to figure out how to fix it yet - short of going into the office and getting charged a fee for their help :( If I figure it out, I'll try to come back here and post.
By the way, the mute answer was lame. It's true.