A choice has to be made about the GUI and I'd like to share the responsability by asking you what do you think :))
You'll find here two screenshots. The important part is the phone number combobox and the call button. So there are two possibilities. The first one is to have two buttons "Call" and "Hang up" (the blue and yellow buttons on the first screenshot) and the second is to have only one button which changes its state from call to hangup (the second screenshot with the little button right to the combobox).
So here is my opinion. I like the two buttons:) but I think that the only case when they'll be enabled together (so we'll see them as they are on the first screen shot) is when someone calls to us. In all other cases, when we are making the call we'll have only one of two buttons enabled, so it's not so pretty:) and it takes a lot of space.
If we have only one button as it's on the second screenshot. We choose a person or write a phone number and the button is in state "Call", when the connection is established the button state is changed to "Hang up" (it may be the same button image as on the other screenshot). When some one calls us a popup appears with photo, user name etc. where we have two buttons and we could accept or decline.
p.s. the colors of the buttons are not yet decided also, so if someone thinks that it's confusing to have blue and yellow for a call and hangup buttons, let me know :))