Innovative use of mobile phones in Africa to save money

In this interview with Vijay Mahajan, the author of Africa Rising, a book I recently read and find very interesting, I read, for example, this:

“People have developed codes for when they call each other. Parents can call their children, using pre-arranged numbers of rings to send messages. For example, three rings means I am ready to pick you up, two rings means meet me at the shopping mall. There is no voicemail, which is one reason why their mobile phone service is cheaper to operate – servers are not required to save the large volume of messages that callers in countries in Europe and North America leave.

Not only have people developed the ability to use mobile phones as a relatively inexpensive medium of communication, in some countries it has become a currency. My driver in Kenya, whose wife was a Massai living in a village some distance away, had three children studying in Nairobi. If the mother wanted to send her daughter a birthday gift she would transfer some of the minutes credit from her mobile phone to her daughter's. In turn, the daughter could go to a commercial establishment and barter some of the mobile phone credit in exchange for a hamburger. The minutes have become a currency.”

What do you do to save money using your mobile phone?