iPhone headphones working for everything except phone calls

Has anyone had this problem?

My 3GS iPhone headphones suddenly cut out during a phone call yesterday and I cannot hear anything through them - tho I can be heard, so the mic is working fine.

However, they still work fine for listening to music etc. so I'm suspecting it is something to do with the iPhone rather than the actual headphones.

thanks for helping

Have you tried replacing the headphones to see if another pair have the same issue?