I happened to notice you haven't received a reply to your post yet. Answering the following questions may help others in the Apple Support Communities get you to a solution:
- What exactly is happening with your iPhone--does it hang up calls unexpectedly, or does the phone become unresponsive?
- Do you get any error messages?
- Are you able to reliably reproduce the issue by doing something specific? If so, what (example, if the phone hangs up calls, does it happen in a certain location? If it's becoming unresponsive, does it only happen when you access one particular app?)