why does voice control keep coming on when i use my ipod

When I'm using my ipod (on the iphone 3 GS), the music will play and then all of a sudden the voice control comes on and interupts the song. I can't get it it stop. I'll click to cancel but then a few seconds later it comes on again and again. I'm using the headphones that came with the phone (white ones). Sometimes I put the phone in a case and strap it to my arm. Other times the phone will just be sitting on the excercise machine but the voice control keeps coming on even though the button isn't being pressed. Any ideas what to do??

Thanks!