There are 326 million wireless subscribers in the U.S. But the battle to take business away from the industry's leaders comes down to just 19 million of them.

That is about how many wireless subscribers have left Verizon Wireless and AT&T Inc. annually over the past three years, according to UBS analyst John Hodulik. It may seem like a lot, but most of those subscribers just go back and forth between the top carriers. The four national U.S. carriers together added only about a net 3.3 million subscribers last year, according...