Do you use advanced config?
If so, here's a passage from baffles' essentials thread:
"Issues with answering your phone when a data connection is open (answer buttons don't do anything)? Looks like a tweak messes up a registry key that affects this. [HKEY_LOCAL_MACHINE\Comm\ConnMgr\Planner\Settings], make sure SuspendResume is set to #777 (not anything containing GPRS). This issue seems to be related to Advanced Config.
source"
I have adv config and thought I had this problem, but turns out i didn't after further inspection.