music while my friends ring me, Help please im desperate!!!!!!!!

Hey folks, thought i was just getting to grips with my iphone 3gs, and my friends tell me that when they ring me they can hear really loud music, how do i switch this off? i dont remember turning it on in the first place, me and technology are about 20 years apart.
They can hear the ringing tone, and they can also hear very loud music,
Please help they are going to stop ringing me