its possible but its not that easy u think
It cant be done inside mobile or when it is moving around...but if u want to tape a call at home u can use this method.
I have done this once.Its easy but u need to find a 2 pin connector that suits the two pin socket provided for headset in nokia .. now have a soldering iron and do solder the wire for the mic and headfone in the two pin connector.
After that connect the mic wire to the mic of computer and headfone wire to line in of computer..when a call comes use some third party software such as musicmatch jukebox setting the source to line in and record..
it worked with me
i did it long back
-Anoop