U.S. authorities have filed a complaint against T-Mobile USA this week, accusing the wireless provider of adding millions of dollars of unauthorized charges onto customer's bills.

The practice is known as "cramming," which can be used in a number of ways to fool people.

Charges were for subscriptions for services like celebrity gossip or horoscopes delivered by text message and often cost $9.99 a month.

T-Mobile USA received 35 to 40 percent of the amount charged, according to Reuters, citing the Federal Trade Commission (FTC).

In a number of cases, customers never authorized the charges, but were signed up anyway without even knowing.

The Federal Communications Commission also confirmed that it was investigating T-Mobile USA for cramming and said it would coordinate with the FTC, according to Reuters.

The FTC asked the court to order T-Mobile to stop mobile cramming, give up any revenues from the practice, and provide refunds.

The company, which is the fourth-largest U.S. mobile phone provided, has a reputation as a price-cutter, though it said in a statement this week it has already stopped billing for the text services.

On June 10 the company announced it would reach out to "crammed" customers to tell them how to request a refund, according to Reuters.

"We are disappointed that the FTC has chosen to file this action against the most pro-consumer company in the industry rather than the real bad actors," Chief Executive Officer John Legere said in an emailed statement, according to Reuters.

Legere said the companies providing the text services are to blame for the cramming.

In the past, the FTC has gone after the smaller companies that provide flirting tips and other services via text message. This is its first action against a wireless carrier for cramming however.

"The FTC's goal is to ensure that T-Mobile repays all its customers for these crammed charges," FTC Chairwoman Edith Ramirez said in a statement.

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