One month after T-Mobile US won a declaratory ruling that could force AT&T and Verizon Wireless to charge lower prices for data roaming, AT&T is trying to reverse the decision.

The Federal Communications Commission order siding with T-Mobile will create a "hopelessly vague and unpredictable" standard for determining whether roaming prices one carrier charges another are reasonable, AT&T said. The telecom filed its challenge late Friday, Politico reported, and the fling was posted online by National Journal reporter Brendan Sasso.

T-Mobile asked the FCC for specific guidance and enforcement criteria for determining whether any given data roaming agreement is commercially reasonable, and the FCC granted the request. The FCC's ruling says that roaming prices can be judged against retail rates, international rates, and resale rates, among other factors. That specific guidance gives T-Mobile a better chance of winning future challenges against rates charged by AT&T and Verizon.

AT&T isn't happy about that. "Responding to a nakedly self-interested plea from T-Mobile for additional leverage in its commercial negotiations with AT&T, the [FCC'S] Wireless Bureau issued a declaratory ruling that purports to 'clarify' the Commission’s rules, provide 'additional guidance' and 'lessen ambiguity,' but has in fact thrown the Commission’s entire data roaming regime into confusion... Rather than provide any meaningful guidance, the Bureau repeatedly tells parties attempting to negotiate agreements that they will simply have to wait and see how the Commission will react in any specific complaint proceeding," AT&T wrote.

The FCC's ruling created "a hopelessly vague and endlessly manipulable standard that allows the Commission, in the role of an external arbiter, to impose arbitrary and unpredictable outcomes in any given case," AT&T wrote.

AT&T also objected to the Wireless Bureau making the ruling without a vote of the five-member commission. Commissioners Ajit Pai and Michael O’Rielly, who form the FCC's Republican minority, also wanted a vote.

AT&T's challenge asks the commission to vacate the ruling.

Verizon Wireless also objected to the ruling last month, saying it would "discourage investment and unfairly advantage one company over other."