Haha that reminds me of a similar hack we discoverd as kids. Australian payphones had an interesting feature where you could place a call without paying however the call would hangup almost immediately upon being answered. There was a split second where you could actually hear each other so we'd usually yell something out and our parents would know it was us waiting to be picked up from the beach or some such.
My Mom's family had a different solution. At 7 o'clock, call. If you let it ring once then it means pick us up now. If you let it ring three times, then pick us up in an hour.
Thinking about it now, Skype gives rings on the caller's side even before it's connected on the receiver's side. In that case, use synchronization. If it takes between 1 and 4 caller rings for the receiver to ring, then a call at 7:00 means "come now" and a call at 7:05 means "come in an hour."