The Department of Justice has approved AT&T’s purchase of Midwest cellular phone company Dobson , the ninth largest carrier in the country. But the Justice Department has required AT&T to sell off assets in Kentucky, Oklahoma, Missouri, Pennsylvania and Texas in order to “preserve competition for residents.” The Dobson buyout, which must also be approved by the Federal Communications Commission, would mean 1.7 million new customers for AT&T ““ most of them rural.