You call a friend. You go through a tunnel, somebody's bluetooth cuts off, the call is dropped. The friend calls you back, but you're already calling him back. No answer, voice mail, whatever, the result is a little bit of rigamarole until you get connected again. The solution is The Protocol. Whoever made the first call is the one who calls back. Simple as that. No confusion, no fuss. Tell your friends. Already doing this? Then please post a derisive comment about how you've been doing this since you were three, how it's obvious to anyone but an absolute moron, and how you're too goddamned cool for school.