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[F]rom Kenya to Colombia to South Africa that cellphones are becoming the truly universal technology. They are the kind of places that have built cellphone towers precisely to leapfrog past the expense of building wired networks that have linked Americans for a century.

The number of mobile subscriptions in the world is expected to pass five billion this year, according to the International Telecommunication Union, a trade group. That would mean more human beings today have access to a cellphone than the United Nations says have access to a clean toilet.

Because it reaches so many people, because it is always with you, because it is cheap and shareable and easily repaired, the cellphone has opened a new frontier in global innovation.