Bicker is 100% correct - AT&T was basically purchased by SBC. Most of the "mergers" were really acquisitions by SBC (i.e. Pacific Bell, Nevada Bell, SNET, Ameritech, AT&T and to an extent BellSouth). It is SBC's old headquarters which is the headquarters of AT&T in Texas. I currently work for AT&T and started out with SBC back in 2000 so I have been around many of these "mergers".
Ah. Your just a PDB then (post divestiture baby). I was with T for a long time, back when SBC was just a backwater RBOC. I saw it all...the good, the bad, and the ugly. Sometimes very ugly. I wonder how many T people are here?