Oh, it was Cingular at announcement and AT&T at launch. Interestingly,

> Cingular chief executive Stan Sigman signed a secretive deal with Apple in 2005 before seeing any designs or prototypes of the phone. Other carriers had been scared off by Apple's reputation for controlling every aspect of its products. But Cingular and AT&T leadership saw the partnership's promise.

Actually, it was the other way around. Cingular bought AT&T Wireless, which was _not_ part of the original AT&T at the time.

Source: https://en.wikipedia.org/wiki/AT%26T_Wireless_Services