The Federal Communications Commission announced today that it is proposing a $100 million fine of AT&T over misleading customers about its “unlimited” mobile data plan. The FCC alleges that the phone service provider purposefully slowed down data speeds for a customer with the “unlimited” plan.

While the company no longer offers the plan, the FCC notes that it still allows anyone who signed up for one after 2007 to renew the plan. In 2011, AT&T started a “Maximum Bit Rate” policy, which capped maximum data for customers who still had the “unlimited” plan if they hit a certain data threshold. If the customer hit that limit, their internet speed would severely slow down compared to customers on other data plans for the rest of a billing cycle.

The FCC says that the company never made customers aware of the “Maximum Bit Rate” policy, which is a violation of the 2010 Open Internet Transparency Rule.

“Unlimited means unlimited,” FCC Enforcement Bureau Chief Travis LeBlanc said in a statement. “As today’s action demonstrates, the Commission is committed to holding accountable those broadband providers who fail to be fully transparent about data limits.”

AT&T said in a statement to USA Today that it will “vigorously dispute” the FCC’s decision.

“The FCC has specifically identified this practice as a legitimate and reasonable way to manage network resources for the benefit of all customers, and has known for years that all of the major carriers use it. We have been fully transparent with our customers, providing notice in multiple ways and going well beyond the FCC's disclosure requirements,” AT&T added.