I find the agendas list and calls list can overlap too. I prefer to use agendas when there is multiple ways of dealing with a person. For people I talk to regularly, I'll use the agendas list because I might bump into them and have an impromptu chat, and prefer this over phone calls. Or if I have the choice of emailing or calling, I might also put that on agendas, as I can have an email thread discussion.
I keep calls list to people I don't have regular contact with or I am only planning to call, not likely to have a face to face, don't have their email address.
Still mulling this over though, and would be interested in other's ideas.