All agencies were believed to have initially pitched for the business, but Initiative reportedly withdrew in September due to a conflict of interest between AT&T and Verizon.
Wendy Clark, vice-president of advertising at AT&T, said that MEC's handling of Cingular Wireless (now AT&T Wireless) had helped the agency's chances.
She said: "Wireless is the growth engine of our business and Mediaedge is credited with much of the success the Cingular property has achieved. They demonstarted to us how they could expand upon that, adding in the other business units to grow our business and maximize our position in the marketplace."
AT&T said it decided to consolidate all of the company's media and planning and buying into one agency because of a desire to "maximise efficiencies". This followed its acquisition of BellSouth at the end of last year, which brought with it the rest of Cingular Wireless that the company did not already own.