Sigman, who this year helped AT&T secure exclusive United States distribution of the Apple iPhone, is succeeded by the company's Group President of Regional Telecom & Entertainment, Ralph de la Vega, effective immediately.

Stan Sigman

Sigman plans to assist the company through the transition, according to AT&T.

Taking over responsibility for the company's regional telecom operations is John Stankey, AT&T's group president of operations and support.

"Stan is one of the industry's most outstanding and accomplished leaders," said Randall Stephenson, AT&T chairman and CEO, in a press release. "Our customers, our employees and our stockholders have benefited tremendously from his unsurpassed vision, expertise and tireless commitment to growing the business. We greatly appreciate his many contributions and wish him all the best in the years ahead."

De la Vega is returning to AT&T Mobility, as he held the chief operations officer post for 2004 to 2006. During that time, he was responsible for leading the integration of the Cingular/AT&T Wireless merger, preparing for the iPhone launch and the introduction of third generation, or 3G, service, according to AT&T.

Sigman's retirement follows that of Edward Whitacre, former AT&T chairman and CEO, in early June.

AT&T Wireless is the largest cellular service provider in the United States with more some 63.7 million subscribers, according to the company.