(Wall Street Journal) — Mavis Nonkongozelo walks up to the Five Sisters convenience store here, then pulls a mobile phone from her bag and a few rands from her brassiere. She is ready to bank. With a few taps on her cellphone, the 34-year old preschool teacher connects to a nascent mobile-banking network aimed at Africa’s new consumers. The saleswoman accepts a 20 rand ($2.94) bill through a barred window and then taps back on her cellphone. Soon, the money is credited to a special no-fee account at Standard Bank, South Africa’s largest. From this corrugated-metal shack outside Cape Town, Standard is breaking from its main business of drawing customers to its branches and automatic teller machines in favor of a low-cost mobile-phone model that is based on proximity to people, like Ms. Nonkongozelo, who have never banked before. The shift says a lot about where banks are placing bets on Africa’s economic growth as a new middle class emerges.