First, you can reset the phone by hitting the Green Phone button, red phone button, and volume up simultaneously.
Second, do you, by chance, have Good for Enterprise installed and running? I had this same problem when I had to use that for work. I tried to work with Good on the issue, but they were convinced it wasn't them, of course. I went through multiple testing phases with it installed and not installed and the problem was consistent (a few times per day) whenever GFE was on the phone.