i had the same problem last week. i just did a hard reset. then yesterday i found this post. seeing i did a hard reset idk if it works.
origional source:
http://forum.ppcgeeks.com/showthread.php?t=40427
"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."