A proposed rule that would reserve certain amounts of low frequency spectrums for smaller carriers at the 2015 FCC spectrum auction continues to come under fire from carriers AT&T and Verizon. Sprint and T-Mobile, they say, have chosen to dig themselves into their current holes, and shouldn't get crutches.

Next year's auction will feature the coveted 600MHz bandwidth -- one that is surely to be snatched up in droves by the juggernauts AT&T and Verizon. In light of how skewed this could make the U.S. mobile market, the FCC has been thinking of setting aside a small amount of the low frequency spectrums for purchase by carriers that cannot compete with AT&T and Verizon's deep pockets.

AT&T is currently the largest carrier in the United States with over 110 million subscribers. Verizon stands next in line with over 100 million, followed by Sprint and finally T-Mobile. The idea of regulating the auction to such an extent has naturally ruffled some feathers in the larger carriers' camps.

"Verizon stressed that it would be perverse and unjust for the Commission to adopt auction rules that subsidize some large multinational companies at the expense of their competitors," Verizon said in a May FCC filing. "T-Mobile and Sprint are large corporations with established, well-financed corporate parents. They and their parent corporations are more than capable of paying substantial amounts to acquire spectrum in the incentive auction if they choose to do so."

AT&T even went so far as to say that it wouldn't participate in such an auction, although the company later retracted its statement saying it would take part.

"Sprint and T-Mobile had their own opportunities to acquire low-frequency spectrum, but they consciously chose not to, purely for reasons of business strategy," AT&T stated in its own May FCC filing.

"AT&T is missing the point," T-Mobile's vice president of federal regulatory Kathleen Ham responded. "Competitions matters. The rules the FCC is proposing are designed to benefit consumers in a wireless marketplace dominated by AT&T and Verizon."

Of course, such a rule may never be implemented if Sprint ends up taking over T-Mobile. Sprint parent company SoftBank's chief executive Masayoshi Son has been making an aggressive appeal to federal regulators to allow Sprint to buy out T-Mobile. By scaling up, Son argued, a combined Sprint and T-Mobile company could offer U.S. consumers a realistic third alternative to AT&T and Verizon. Without more resources to snag up spectrum and expand, Sprint and T-Mobile could be stuck in a rut for years to come.

The deal, however, has stiff opposition from U.S. regulators. Both the FCC and the Department of Justice's antitrust division have expressed concern at consolidating the U.S. market from four to three carriers and approval from both institutions is required.