Save a number during a phone call

How can I save a number during a phone call? The scenario is that I'm calling someone to get the phone number of someone else. If I just type the number in the dialer, it disappears when the call ends. Opening some kind of notepad application is cumbersome both saving and retrieving: tap home, tap applications, find the application, start a new note, type number, end call, copy number, paste number into the dialer (how to do the last step is another question)