AT&T today said that Stan Sigman, CEO of AT&T Mobility, is retiring effective immediately. Group President Ralph de la Vega has assumed the CEO role, returning to the mobile unit he oversaw the operations of as chief operating officer.

AT&T did not name any specific reasons for Sigman’s retirement today after working for AT&T for 42 years. At age 60, he is five years younger than former CEO Ed Whitacre, who retired in June after guiding the AT&T’s acquisition of fellow RBOC BellSouth.

De la Vega has been group president of the regional telecom and entertainment since 2006. He was chief operating officer Cingular Wireless from 2004 to 2006, overseeing the massive integration of AT&T Wireless with Cingular and the launch of Cingular’s first 3G networks. De la Vega’s new role as CEO of the largest wireless operator in the U.S. effectively takes his name out of circulation for the Sprint CEO job, which came open on Monday when Gary Forsee stepped down.

Taking over de la Vega’s former post is John Stankey, AT&T group president for operations support, who will fold the regional telecom operations into his group.