This post is in partnership with Time. The article below was originally published at Time.com.

By Victor Luckerson, TIME

The Federal Trade Commission is suing AT&T ATT for allegedly misleading customers by slowing data speeds for wireless customers who had unlimited data plans but went over a certain usage point, the agency announced Tuesday.

According to the FTC, AT&T did not properly inform customers who had unlimited plans that their speeds would still be lowered after they exceeded certain data thresholds in a given month. Speeds were reduced by as much as 90 percent in some cases, making basic phone functions such as web browsing and watching video almost impossible, the FTC said.

“AT&T promised its customers ‘unlimited’ data, and in many instances, it has failed to deliver on that promise,” FTC Chairwoman Edith Ramirez said in a statement. “The issue here is simple: ‘unlimited’ means unlimited.”

AT&T throttled speeds for 3.5 million customers at least 25 million times, the FTC alleges, while it also said that customers who canceled their contracts due to the lowered speeds still had to pay expensive termination fees, the FTC alleges.

In an emailed statement, AT&T senior executive vice president and general counsel Wayne Watts called the FTC’s suit “baffling.”

“The FTC’s allegations are baseless and have nothing to do with the substance of our network management program,” Watts said. “We have been completely transparent with customers since the very beginning. We informed all unlimited data-plan customers via bill notices and a national press release that resulted in nearly 2,000 news stories, well before the program was implemented. In addition, this program has affected only about 3% of our customers, and before any customer is affected, they are also notified by text message.”

AT&T no longer sells unlimited data plans to new customers and has been trying to phase out the service for years, along with many other major carriers. The company announced in 2011 that it would begin throttling the data speeds of its heaviest users on a regular basis.

Wireless carriers’ practice of slowing speeds for their heaviest unlimited users has also caught the attention of the Federal Communications Commission. “Wireless customers across the country are complaining that their supposedly ‘unlimited’ data plans are not truly unlimited, because they are being throttled and they have not received appropriate notice,” said an FCC spokesperson Tuesday. “We continue to work on this important issue, including with our partners at the FTC, and we encourage customers to contact the FCC if they are being throttled by AT&T or other cellular providers.”