Senate lawmakers said Tuesday that they sent a letter to AT&T asking for more information about a lawsuit alleging the wireless giant systematically overcharged smart phone customers.

Sens. Amy Klobuchar (D-Minn.) and Herb Kohl (D-Wisc.) wrote AT&T Chief Executive Randall Stephenson asking a series of questions about its billing practices.

Kohl is chairman of the Senate Judiciary committee and recently held a hearing on AT&T’s proposed merger of T-Mobile, to explore whether the merger presents antitrust concerns. Klobuchar is a member of the committee and has supported rules that prevent bill shock from data overcharges and limits to penalties on early contract cancellations.

AT&T has disputed allegations by customer Patrick Hendricks, who filed his class-action suit in the U.S. District Court of Northern California. According to Hendricks, AT&T systematically overstates Web server traffic for smart phone users by 7 to 14 percent.

He hired an independent consulting firm to study billing of AT&T customers, according to the suit.

Klobuchar and Kohl asked AT&T to respond to the charges. It also asked for the company to specify how it ensures the accuracy of bills and to provide an estimate of how many errors occur in their charges for data and other services. They asked how many people exceed their monthly data caps and the amount such customers are billed for excess data usage during the three months preceding the date of this letter.

“The answers to these questions will be important as we consider competition in the cellphone industry, including the proposed AT&T/T-Mobile merger,” the lawmakers wrote. “And, whatever the validity of the allegations of overcharging, our goal is to ensure going forward that consumers can have confidence that their bills for data service are completely accurate and reliable.”

AT&T spokesman Michael Balmoris said customers can monitor their data use through the company’s Web site and by dialing *data# from their wireless device.

“We are confident that our billing systems are accurate,” he said. “We properly bill for all data that our customers send and receive, including e-mailing, downloading applications, browsing the Web, downloading a video or streaming music.”

This post has been updated since it was first published.