Randall Stephenson (left) is replacing Edward Whitacre, Jr. as chief of AT&T.

Randall Stephenson, once CFO and most recently the company’s chief operating officer, is now the chairman and chief executive officer of telecom giant AT&T. Stephenson took over at the helm on Sunday after Edward Whitacre Jr., who spent 17 years as Chairman and CEO, officially retired.

Stephenson is well known for helping Whitacre mold the new AT&T, formerly SBC Communications. He began his career with Southwestern Bell Telephone’s information-technology organization in Oklahoma. He later held a series of posts in finance, leading up to controller and then CFO of SBC Communications, which already had Whitacre at its helm.

During Stephenson’s time as CFO he helped reduce SBC’s debt from $30 billion to almost zero in early 2004, putting SBC in position to acquire AT&T Wireless, AT&T Corp., and BellSouth. He was then promoted to COO of the new AT&T in 2005.

At a Lehman Brothers conference last week, Stephenson, who has been officially pegged to become CEO since late April, said he would be focused on the biggest challenges that AT&T faces, including competing with cable operators in the residential markets since on Internet and telephone services. He also said he has big expectations for Apple’s iPhone, since AT&T is the sole carrier of the device, which is slated to be released in coming months.