"Who the f___ are you anyway, EFF?"

John Legere delivered his F-bomb Thursday with a wry grin, but behind the snark, the chief executive of T-Mobile was clearly frustrated.

The target of Legere's ire — a consumer advocacy group known as as the Electronic Frontier Foundation — has been a thorn in T-Mobile's side all week. On Monday, it published a study accusing T-Mobile of interfering with its customers' mobile videos, heightening scrutiny over the wireless carrier's Binge On program just days before the company is expected to meet with federal regulators on the issue.

— John Legere (@JohnLegere) January 7, 2016

T-Mobile has been waging an increasingly vocal PR battle ever since it activated Binge On for all of its customers. Binge On allows you to watch dozens of online streaming video services without having that consumption count against your monthly data cap. Meanwhile, however, some non-participating video services say they have been negatively affected by Binge On, even though  T-Mobile says Bi lets customers watch significantly more video overall.

T-Mobile has pitched the service to consumers as a way to stretch their data usage, and is entirely optional. But opponents say the tactic amounts to a violation of net neutrality, in part because it singles out a particular type of Internet traffic (video) and degrades it.

YouTube, which is not a T-Mobile partner, objected to the practice last month as a form of illegal "throttling," fueling the carrier's critics. In the wake of those reports, some consumers reported that Binge On was indeed restricting their download speeds from YouTube and the gaming service Twitch — and that the situation improved when they deactivated the feature, which is enabled for all customers by default. (Other consumers have leapt to the company's defense on social media, and Legere has been promoting their responses.)

EFF's report attempted to verify some of these claims by running its own tests. Its report Monday compared download speeds on T-Mobile's network while Binge On was activated and when it was deactivated, finding that when the service is turned on, T-Mobile slowed video traffic to roughly 1.5 Mbps. That's compared to 5 Mbps when Binge On was disabled. That could explain why some users were seeing lags in their service.

Legere fired back at those findings Thursday morning with an initial video and blog post, calling the attempts to portray Binge On as a net neutrality violation "semantics" and "bulls__t." But he reserved special treatment for EFF in a Q&A on Twitter later in the day.

"Why are you stirring up so much trouble?" he asked, after asking who EFF was. "And who pays you?"

For the uninitiated, EFF has been involved in some of the largest tech policy fights in Washington and around the country. It defends individuals in lawsuits concerning technology, intellectual property and surveillance, and it played a role in the Internet blackout day to oppose the legislation known as SOPA and PIPA. Its efforts have also been directed towards net neutrality — its attack on T-Mobile being the latest example. The organization's financials are here. As for its response to Legere?

Ultimately, however, whether Binge On is allowed to stand will be up to regulators, who are expected to meet with T-Mobile by Jan. 15.

Update: Legere has now addressed EFF again, clarifying some of his remarks.