i dont think you can completely disable it during a call.
my solution was to change the ring type/tone to something a bit more tolerable. there are a few other threads dealing with this issue (seach attenuation).
here is what i did..
i changed the ringtone by editing the registry. i am sure you could just delete the tone all together as well.
http://forum.ppcgeeks.com/showthread.php?t=41178
HKCU/ControlPanel/Sounds/InCallBuzz
I changed the Sound value from Notify.wma to Default.wma
So now it just beeps 3 times when a text comes in while on a call. "
more threads
http://forum.ppcgeeks.com/showthread.php?t=116369
http://forum.ppcgeeks.com/showthread.php?t=113407
http://forum.ppcgeeks.com/showthread.php?t=100387