Baffled me too. While not in a phone call,
1. If you are currently editing a memo, save it.
2. kill the app (hold the Android Home button, tap Task Manager, and if Memos is showing, tap its Exit.)
3. run the Memos app again.
You should now see your history of memos, because in this case it starts from a different Activity.
This Memos app starts up quickly, which is essential while in a call. A program like Evernote is too slow to start. I miss my 1996 NTT phone that could record my side of the conversation - I'd just repeat what they said: "that was 411-4596, right? thank you very much!" Now would that be such a difficult app to make? I would pay.
$10 if it could be started during the phone call by pressing the camera button. $20 more if if would forward the audio file automatically to Google Voice or Dragon or Evernote after the call. Any takers on my challenge?