T-Mobile will pay at least $90 million for failing to stop bogus charges placed on customers' bills — a practice known as cramming, the Federal Trade Commission and Federal Communications Commission announced today.

In a complaint filed back in July, the FTC said that T-Mobile made "hundreds of millions of dollars" by charging customers for premium SMS subscriptions. These services — which ran up to $9.99 per month for things like flirting tips, horoscope information, or celebrity gossip — were added to T-Mobile users' bills by scammers without the customer's authorization.

When customers complained, however, T-Mobile continued to bill some users for years, the FTC said. The carrier made anywhere from 35 to 40 percent of the total amount charged to consumers.

Now, T-Mobile has agreed to fully refund customers for the unwanted third-party charges — to the tune of no less than $90 million. In addition to the refunds, T-Mobile will be required to pay $18 million in fines and penalties to the attorneys general of all 50 states and the District of Columbia and $4.5 million to the Federal Communications Commission.

This is the second-largest mobile cramming settlement ever handed down, only trailing AT&T's $105 million settlement with the FTC and FCC this October, Travis LeBlanc, chief of the FCC Enforcement Bureau, said on a call with reporters Friday afternoon. He added that cramming is a significant problem, which has cost consumers "hundreds of millions of dollars."

"T-Mobile was in bed with the crammers," LeBlanc said. "Crammers are predators; they're a threat to consumers everywhere. The FTC has received many thousands of complaints about cramming."

The settlement requires T-Mobile to contact all affected customers to inform them of the refund program and claims process. T-Mobile did not immediately respond to a request for comment about the settlement, but in July called the charges "unfounded and without merit."

But the FTC's complaint alleges that so many T-Mobile customers complained about the charges that it should have been obvious to the company that users had not authorized them. Furthermore, T-Mobile's phone bills were so complicated that it was "nearly impossible" for users to decipher what the excess charges covered. They were hidden in a section called "Usage Charges" and sometimes described by indecipherable letters and numbers, like "8888906150BrnStorm23918."

The settlement also requires T-Mobile to get express informed consent before placing third-party charges on users bills in the future. The company must also ensure consumers are notified of any third-party charges on their bills and give them the option to block these charges.

Earlier this week, meanwhile, the Consumer Financial Protection Bureau (CFPB) sued Sprint for illegally billing wireless customers tens of millions of dollars in unauthorized third-party charges - like monthly subscriptions to services like horoscopes, sports scores, and ringtones. There are also reports that Sprint, too, will face a $105 million penalty, but no announcements have been made.