A "hack" would be to switch the sender and receiver addresses on the envelop; and mail without a postal stamp. What is the postal service supposed to do with an envelop missing the stamp? Yes, you are right. Return it to the sender.
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."