When you mention that music plays over the call, I just want to be clear and ask if you mean music you have downloaded to the phone (songs), or do you mean the ringtone that played when the phone rings?
I have checked my resources and I also wanted to ask if you are using headphones when this happens? Does it happen on every call?
If you are using headphones when this happens, you are able to search the market for a headset blocker. If this is the issue you are having, the download will disable the music from playing when on a call.
Please post back with your results.