T-Mobile on Monday agreed to pay $40 million to settle an investigation into problems with rural call delivery — including the inserting of “false ring tones” that gave the impression that undelivered calls simply went unanswered.

The carrier will also enter into a compliance plan to “prevent future violations.”

The Federal Communications Commission said its Enforcement Bureau opened the probe after it received complaints from T-Mobile users who were unable to reach consumers served by three carriers in Wisconsin.

The commission continued to receive complaints even after the carrier reported that the issues were “resolved,” and officials said complaints to the company showed call delivery problems “in at least seven other rural areas.”

Failing to correct rural call problems, the agency noted, could violate the federal Communications Act.

“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 statement.

The investigation, the agency added, also led T-Mobile to acknowledge inserting false ring tones — which purport to indicate that undelivered calls went through — into hundreds of millions of calls, in violation of FCC rules.

T-Mobile told Bloomberg in a statement that the “ringtone oversight” was unintentional and corrected in January 2017.