NAD, an ad industry self-regulatory council, said Monday that it is recommending that T-Mobile discontinue its "best unlimited network" advertising claims, responding to a challenge by its rival.
The news comes on the heels of T-Mobile's $26 billion deal to acquire Sprint.
NAD said T-Mobile "did not provide evidence that its network is superior in providing talk and text mobile services, or in providing high-speed data more reliably or to a greater coverage area."
Various features that do seem superior are elements of T-Mobile unlimited plans, not the entire network, "and do not support the 'Best Unlimited Network' claim," NAD added.
T-Mobile had provided data from independent sources to back up its claims of superiority in speed and other areas, according to NAD, but the group decided that the data couldn't "match the breadth" of "Best Unlimited Network."
Representatives for T-Mobile and AT&T did not immediately respond to requests for comment, but NAD said T-Mobile plans to appeal.
NAD's findings are only suggestions about what the company should do. T-Mobile is not legally required to make any changes.
All the same, the NAD has become a frequent forum for spats between large telcos.
In 2013, AT&T filed a complaint through NAD after T-Mobile said its network provided 50 percent more bandwidth. "Boy ATT sure are poor sports," Legere said in a tweet at the time. "Keep whining, we'll keep enhancing our network. Nationwide 4G #LTE before the end of year, baby!"
In 2015, T-Mobile used NAD to get Sprint to drop the terms "brand new," "all-new" and "America's Newest," as well customer satisfaction claims, from its marketing.