In the developed world (except the US and Canada) voice calls are cheap and inclusive. Kids in the UK have phones for £10 a month, happily funded by parents, and they either hit a limit and have to top up or have enough inclusive minutes.

The developing world isn't going to be so quick to hand over its hard earned money to big corporates. The apps designed around missed calls as a communicative action are genius: you could use it in a variety of services: "Miss call Dominoes and we will send you your regular order within 30 minutes.... Could cause havoc too :) ( maybe need a railways return call to double check the order, on second thoughts...)

Maybe we are missing a trick here with consumers? A missed call is faster and more convenient than a text message... Would be a great marketing or polling tool