When someone is called while he's already talking on Skype: Very frustrating for both sides because the call is stopped very quickly with a "busy" message on one side (Don't always bother to look at the message board), Maybe better have a short "busy" signal before stopping the call?. Also, the existing solution does not seem to always work, i.e. the other party keeps receiving a call signal when he's still talking to someone else.
A solution might be to have the online icon turm into a busy line icon when someone you want to call is already on the line, only to turn back into the normal online mode when he hangs up. This would avoid a lot of frustrations on both sides.