Here's a really interesting look from Younghee Jung at the global phenomenon of multi-SIM phones that can talk on multiple networks at once. Some people get these SIMs because they want to take advantage of low-cost calling within an single network (which means that you have to keep track of which network each person in your address book uses!). Others use it to establish priority -- a business man who has a "private number" for his best customers that he always answers. Sometimes, it's just a way to get a bargain, loading up prepaid minutes on different SIMs depending on who's got the best deal. A sketch from a Liberian refugee camp in Ghana depicts an "ideal phone" that holds four SIMs.
Many mobile network operators offer cheaper rates for inter-network calls, especially in markets where competition among network operators is high. Highly cost-conscious consumers naturally get multiple numbers for cheaper calls. While it may not take too much effort to acquire the new number itself, this comes at a cost of efforts and skill: Remembering, or identifying who in your social network has the number belonging to a specific network operator. People develop a tactic, such as indicating the network operator in the name stored on the phonebook. This is not an exclusive behavior only for the developing economies, however. When the 3G network was newly introduced in Japan several years ago, many Japanese consumers also owned two numbers, one from 3G for cheaper messaging & data connection, another from existing network for cheaper voice calls.Use of Multiple Mobile Phone Numbers (part 1) (via Kottke)