One pilot program allows about 100 farmers in South Africa's northeast to learn the prevailing prices for produce in major markets, crucial information in negotiations with middlemen.

Health-care workers in the rural southeast summon ambulances to distant clinics via cellphone.

One woman living on the Congo River, unable even to write her last name, tells customers to call her cellphone if they want to buy the fresh fish she sells.

"She doesn't have electricity, she can't put the fish in the freezer," said Mr. Nkuli of Vodacom. "So she keeps them in the river," tethered live on a string, until a call comes in. Then she retrieves them and readies them for sale.

William Pedro, 51, who deals in farm and garden plants, said he tried for eight years to lure customers to his nursery in a ragtag township near George, a resort town on South Africa's southern coast. Only when he got a cellphone two years ago, he said, did his business take off.

"White people are afraid to come here to my place in the township to buy plants," Mr. Pedro, who is of mixed race, said as he stood outside his makeshift greenhouses. "So now they can phone me for orders and I can deliver them the same day."

Hamadoun Touré, development director for the International Telecommunication Union, said the economic blessings of cellphones were magnified in the developing world.

"What is the alternative?" asked Mr. Touré, whose agency was founded in the days of the telegraph and is now part of the United Nations. "Somebody may have to leave work, travel for days, spending much more money" just to pass on a message.