What version of Android OS are you on? I have SGS2 on Android 2.3.3 and when you are on a call, there is an menu option to jump to memo during the call. The menu option is not there when you hang up. Hit menu, Memo and it jumps to the memo app while the call is still on in the background accessable by the notification bar where you can hangup, place on hold etc directly from the notification bar. Try it while you call voicemail.
Easy to overlook. I have used this a few times. Once you have the text, you can cut/paste it after the call?