iPhone 4 phones calls interrupted by music

When I'm making a phone call I will suddenly be cut off and the Phone starts to play the music that is on the device. I'm not doing anything to the phone, not pressing buttons shaking it or anything. I can be talking to somebdoy and then suddely they are gone and the music is playing. Sometimes it takes me four or five attempts to get the call back , as the music will interrupt the dialling and can cut in at any time.

I checked and the music app is not active after this happens. The iPhone is hardly usable as a phone noew the problem has got so bad. Anybody know if this can be fixed?

My next move will be to delete all the music on the device, pretty poor for a $600 phone.