For the many millions of people in emerging economies who don’t have telephone lines, mobile-communications networks could bring immense benefits. Low-income customers could not only make telephone calls, perhaps for the first time, but also gain access to such lubricants of economic growth as modern banking and information services—things they can’t afford at present.

Some mobile operators in emerging markets have already built networks to serve the affluent, and other companies plan to follow. But so large is the number of low-income people that they too could potentially be served at a substantial profit on even the slimmest of margins. Why haven’t mobile operators extended their networks to cover these potential users? The answer is mainly that the operators cannot yet build and run their networks at a price the masses can afford.

Then network operators are hardly the only party interested in cheaper mobile networks. Governments want the economic growth that mass mobile communications promise. Banks and other companies want networks to be extended to provide profitable services to a huge, previously inaccessible market. And local entrepreneurs in poor regions would welcome the opportunity to act as franchise managers of the mobile last mile.

In India, low-cost mobile...