Haha, this is the digital version of the Indian phenomenon of 'missed calls', used as 1-bit 0-cost notification mechanism. It's become such a cultural artifact, that big companies are now advertising numbers you can 'missed call' and get a callback from.
It's not Indian specific either. I've seen it in a number of places throughout the world. It works anywhere that does not charge to receive calls (i.e. any sane place outside of the U.S.A.) as long as the caller doesn't get billed for cancelling.
You can get more than 1-bit of information as well if you sync the clock on your phone with the recipient. That gives you approximately 3.3 bits of information if you use the minute modula 10. This only works if you previously agree upon a meaning for values (Mod 0: Yes, Mod 5: No, etc).
Those of us old enough to remember AT&T before the breakup probably had a similar system with our parents. Call, let it ring once and hang up. Repeat. Wait for mom to call you on the family phone.
You can get more than 1-bit of information as well if you sync the clock on your phone with the recipient. That gives you approximately 3.3 bits of information if you use the minute modula 10. This only works if you previously agree upon a meaning for values (Mod 0: Yes, Mod 5: No, etc).