The Federal Trade Commission is sending refund checks totaling almost $20 million to more than 617,000 T-Mobile US Inc. customers, as part of a mobile cramming case settled in December 2014.

The checks, with an average amount of $32, will be sent to T-Mobile customers who incurred unauthorized third-party charges on their mobile phone bills, but did not participate in the telco's refund program, the FTC said Feb. 1. Recipients were advised to deposit or cash the checks within 60 days.

In July 2014, the FTC sued T-Mobile, alleging that the latter received anywhere from 35% to 40% of the third-party charges. After holding talks with the FTC, T-Mobile eventually agreed to pay $90 million to settle the case, which involves the FTC, all 50 U.S. states and the District of Columbia, and the FCC. T-Mobile also agreed to turn over to the FTC any remaining undistributed funds under the settlement, which are now used for the refund checks.

The FTC is also refunding more than $88 million to AT&T Inc. customers in a separate mobile cramming case.