T-Mobile CEO John Legere will step down next spring, but until then will be focused on making sure his company’s $26 billion merger with Sprint is finalized, including an upcoming court battle with several states attorneys general.

T-Mobile’s John Legere

Both the Federal Communications Commission and the Department of Justice have approved the proposed T-Mobile-Sprint merger. The states, led by New York and California, are seeking to block it, and the trial is set to begin in December in a federal court in New York.

Mike Sievert, T-Mobile’s president, COO and a board director, will take over as CEO after Legere steps down on April 30.

“In the months ahead, my focus will be on ensuring a smooth leadership transition and continuing to work closely with the board and Mike to complete the Sprint transaction,” Legere said. “This merger will create the New T-Mobile – a company that is uniquely positioned to continue disrupting the wireless category – and beyond. This marks the beginning of a dynamic new chapter for T-Mobile.”

According to a Bloomberg report, T-Mobile has said in a court filing that the antitrust expert hired by the states seeking to block its merger “rigged” his analysis of stock prices in a report that will be used as evidence in the trial.

In the filing, T-Mobile and Sprint argue that Carl Shapiro, a University of California Berkeley professor, hand-picked trading data and ignored historical stock movements that undermined his theory about why the merger should be blocked on antitrust grounds.

The filing said the expert’s report should be blocked from trial because it’s unreliable and contradicted by share movement in the industry when the merger was announced and when the suit was filed, according to Bloomberg.

During the past two years, Sievert has worked with Legere on T-Mobile’s merger planning, integration and regulatory approval initiatives, with the goal of “creating the New T-Mobile by successfully closing the pending acquisition of Sprint,” according to T-Mobile.