The Federal Communications Commission and T-Mobile agreed to a $40 million settlement after an investigation revealed the Bellevue, Wash.-based wireless carrier played a fake ring tone on “hundreds of millions of calls.”

T-Mobile allegedly faked the ring tone on calls to rural areas with poor reception, buying more time to connect with local providers in areas the carrier does not serve, as TechCrunch notes. Implying that a call has gone through when it hasn’t is prohibited because it could lead customers to hang up, thinking there isn’t anyone available on the other end.

Update: A T-Mobile spokesperson provided the following comment about the settlement:

T-Mobile is committed to all of our customers across the country. Our actions have always been focused on better serving our customers and the ringtone oversight, which was corrected in January 2017, was unintentional. We have settled this matter — and will continue to focus on our mission to change for wireless for good for consumers everywhere.

The FCC opened the investigation after consumers in Wisconsin complained their calls were not going through. T-Mobile told the FCC the problems had been resolved but complaints continued.

“False ring tones are a problem on calls to rural areas and are a symptom of the problems of impaired quality and completion of calls to rural areas,” the FCC said in a consent decree issued Monday. That can “cause rural businesses to lose revenue, impede medical professionals from reaching patients in rural areas, cut families off from their relatives, and create the potential for dangerous delays in public safety communications,” the FCC says.

The consent decree requires T-Mobile to come up with a compliance plan with FCC rules within 90 days and provide regular reports on the issue over the next three years.

“It is a basic tenet of the nation’s phone system that calls be completed to the called party, without a reduction in the call quality — even when the calls pass through intermediate providers,” FCC Chairman Ajit Pai said in a press release. “The FCC is committed to ensuring that phone calls to all Americans, including rural Americans, go through.”