U.S. Cellular is the country's sixth-largest wireless carrier, with 5.8 million devices on its network. As wireless data speeds become more and more important to customers, smaller carriers like U.S. Cellular are finding it hard to compete against the biggest carriers, Verizon Wireless and AT&T, and it's been losing high-paying customers at a slow but steady rate for the last three years.
The acquisition is subject to regulatory approval, and is expected to close in the middle of next year.
Also on Wednesday, U.S. Cellular said its third-quarter net income dropped 43 percent, as the company subsidized sales of new smartphones.
U.S. Cellular earned $35.5 million, or 42 cents per share, down from $62.1 million, or 73 cents per share, in the same quarter last year. Revenue rose 3 percent to $1.14 billion.