When I have to make a series of calls, I use Google Voice to call out via my Gizmo5 client so I can use my computer headphones and keep my hands free for typing.
As I wait for the call to connect, I create a new Evernote note for the call and open it in its own window.
In the Evernote client, I search Evernote for previous calls I've made to the same vendor. When I finally get done holding for the next available operator, I have all the information at my finger tips; in fact I have to wait for them to pull up the same information from their computer.
I have found some of the people I talk with to be a bit thrown by the idea that I have all my notes of previous conversations before me. It makes it harder for them to, well, "handle" me.