T-Mobile plans to buy “substantially all” of U.S. Cellular’s wireless operations in a deal worth about $4.4 billion, the two companies announced Tuesday.

The deal would expand T-Mobile’s presence to millions of people in “underserved rural areas,” the company said. Under the terms of the deal, U.S. Cellular will retain 70 percent of its wireless spectrum and keep its cellphone towers but lease space on at least 2,100 of them to T-Mobile.