Washington state regulators have issued a penalty of $41,400 against Frontier Communications Northwest Inc. for overcharging customers for returned checks, according to a news release from the state’s Utilities and Transportation Commission.

The regulatory agency found that Frontier committed 414 violations of the state’s consumer protection rules by improperly charging customers a higher fee than allowed for returned checks.

After its purchase on Verizon Northwest Inc. in 2010, Frontier became the second-largest landline telephone service provider in Washington. The company serves phone lines in Lynden and Mount Vernon, as well as in Redmond, Kirkland, Everett, Bothell, Woodinville, Wenatchee, Kennewick, Pullman, Chelan, Richland, Naches, Westport, Anacortes, Newport, Oakesdale, Republic and Camas-Washougal, according to the UTC.

Frontier has 15 days to pay the fine (which was issued Aug. 20), request a hearing to contest the violations, or request mitigation of the penalty. The company is not allowed to pass penalty costs to customers through rates.

The violations were found during an investigation of returned check fees charged to customers from Aug. 1, 2010, to March 31, 2012. UTC staff found Frontier was improperly charging $20 and $25 fees for returned customer checks, higher than the $15 the company is authorized to collect.

Commission staff informed Frontier of the overcharges after receiving a consumer complaint.

The company has provided refunds or credits to customers who were overcharged and changed its billing practices to ensure the proper amount is charged to customers for returned checks.

Headquartered in Stamford, Conn., Frontier Communications serves almost 3.2 million customers in rural communities, suburban markets and smaller cities across 27 states. It is the fifth largest phone company in the U.S.