When asked directly why AT&T had so much unused spectrum, Stephenson said that his company currently lacked the "20MHz of contiguous unused spectrum" to launch its 4G services in many parts of the country.

"To make the move to 4G ... we have to have clear blocks of unused spectrum," he said. "Right now we don't have enough spectrum to launch in West Virginia, for instance."

Panelists and senators also expressed concern that the merger would lead to a wireless duopoly dominated by AT&T and Verizon, which would control about 80% of the wireless market if the merger were to pass. Hesse in particular said that his company was in serious danger of becoming a takeover target if that happens, meaning that two large wireless companies would have near-total control of the market.

"The wireless industry thrives on competition, which, in turn, drives investment, innovation, consumer choice, job creation and U.S. global leadership in wireless communications," Hesse said. "If AT&T is permitted to devour one of the two remaining independent national wireless carriers ... the U.S. will go backwards toward last century's Ma Bell."

Stephenson said that even though T-Mobile would no longer be in the wireless mix, AT&T would still face strong competition not only from Verizon but also smaller players such as Leap and MetroPCS. Sohn scoffed at this, however, and said the idea that MetroPCS could effectively compete with AT&T was like saying mom-and-pop stores could effectively compete with Walmart.