T-Mobile CEO John Legere speaks on the floor of the New York Stock Exchange, April 30, 2018.

T-Mobile US told the U.S. Federal Communications Commission on Monday it would not increase prices for three years, with few exceptions, if it gets approval to buy rival Sprint for $26 billion.

In a letter to the FCC, T-Mobile Chief Executive John Legere asked the government to move forward "expeditiously" in reviewing the merger of the No. 3 and No. 4 wireless carriers, and attempted to allay fears the deal would mean higher prices.