The Conversation area (left side) of the main window is where all conversation activity occurs. By paying attention to the Incoming, Active, and Pending (Hold) conversations, you can easily manage multiple conversations at once.
One active conversation
Keep in mind that the conversation in the Active area is the one you’re addressing at the moment. To answer an incoming call or to move a pending call to the Active area, just double-click it.
Multiple active conversations (phone and IM)
You can maintain multiple active conversations at the same time, provided only one of them is a voice conversation. For example, you can accept an incoming call while you're maintaining two other instant messaging (IM) conversations. In this case, whichever IM conversation you are addressing at the moment moves to the Active area, and the voice conversation would still be active (indicated by the red arrow) even though it moves to the Pending area on your screen.