This would have given the impression that the calls were going unanswered rather than not connecting.

The settlement was negotiated by the Federal Communications Commission.

T-Mobile said the issue was "unintentional" and had been "corrected" in January 2017.

The FCC banned false ringtones in 2014.

The FCC also accused the mobile phone network of failing to fix problems with completing calls in rural areas.

In a statement, FCC commissioner Mignon Clyburn said the practice may have affected "billions" of calls over a period of several years.

However, Ms Clyburn also criticised FCC chairman Ajit Pai's office for negotiating a settlement that didn't include consumers.

"Prior consent decrees have included direct-to-consumer benefits, such as refunds or discounts, or notifications to customers who have been impacted," she said.

"Despite demonstrating a clear and tangible consumer harm, in this consent decree, consumers are treated as a mere afterthought."