Nobody can hear me if...

I tried researching about this and got no response. My problem is that whenever I am on a call and my finger slides over the headphone jack the person on the other end can not hear me at all! The phone also then makes a sound, the best way I can describe that sound is when you cover or tap a microphone, that sound. This is frustruating and was wondering if it happens to others. I tested it just now with my wife on the other end and she could not hear me at all when the headphone jack was covered by my finger. Any suggestions?