The parent company of Alltel, a mobile telephone network serving rural customers in six states, has agreed to sell the business to AT&T for about $780 million, Atlantic Tele-Network announced Tuesday.The all-cash transaction will give Alltel customers access to a nationwide 4G mobile network, a larger selection of mobile devices and additional retail locations, said Michael Prior, Atlantic Tele-Networks CEO. Many of the company's employees will benefit from new career opportunities at the larger company, he added.