The service is being launched where access to traditional banks is poor.
"We have launched this service in South Africa in 2005," Dare Okoudjou of MTN's mobile phone banking department told AFP.
"But it wasn't a full bank account with the mobile phone at the primary access, it was very focused on key basic services, such as money transfer," he said of the South African service.
to offer a fully-fledged bank account on mobile phones called MTN MobileMoney which will allow users to pay for purchases or check balances. A credit card will be optional.
The service will be "a convenient, secure and affordable way for MTN subscribers to send money, buy airtime and pay bills using their cellphone", according to the company.
Among the 20 other coountries to receive the service are Uganda, Nigeria, Cameroon and Ivory Coast which have a combined 90 million mobile phone users.
MTN has signed a 9.7-million-dollar (7.5-million-euro) deal with Fundamo, a South African-based specialist provider of enterprise mobile financial services software.
Both British mobile phone giant Vodafone and France's Orange are active
in the market.
"In Benin, banking penetration is about one percent of the population, compared to 1,5 million telecom lines," or 18 percent of the total population, said Okoudjou.
A recent study by telecom analyst firm Berg Insight states
the service states could be potentially used by 913 million people by 2014, against 20 million today.