To win approval for their $26-billion deal, T-Mobile and Sprint are trying to convince regulators that plenty of competition would remain even if they merge. One of the telecommunication companies’ strategies is to redefine the wireless market to show there are actually more than four competitors today—making a merger of the third- and fourth-biggest mobile providers less troublesome. The two telecom giants are hoping this argument gives regulators cover to say that consumers wouldn’t be harmed by the merger.

The new competitors...