Cell phones are banned in Peru's banks. I think it has to do with the notion that you can plan a robbery using a cell phone. There are no facts or statistics to support any ban on cell phones but you know how our countries are.
As far as the high number of cell phones in poor countries, it's the same story in Peru. Getting a landline is extremely expensive and, in many cases, impossible since the phone company has no lines reaching the poorer areas. People who drive taxis in Lima, for example, now have a "contact" number to do business. So, a cell phone becomes a lifeline. With the prepaid system, it's even affordable. And in Peru, the caller pays for the call so a cell phone user doesn't waste minutes if they receive a call, only if they make it. So you can call the taxi driver a hundred times and they don't waste their minutes.
Faucett. La primera linea aerea del Peru.