I had a similar problem.

Set your phone so it should vibrate, and initiate a call to your phone.
As the phone rings, press the top, and then the bottom of the phone, and see
if it vibrates when you do this.

I found that was happening with mine, so it appeared to be a loose
connection. Had to take it to a service point, where they sorted the
problem.

I know his won't help YOU to fix your phone, but it will/may give an
indication as to where the problem lies.

Good Luck