The company you think of as AT&T is really a re-branded SBC and has only been AT&T for 5 years.
The company you think of as a re-branded SBC are reassembled parts of the AT&T that the DOJ broke up in 1984. 4 of the 7 baby Bells and the AT&T long distance company. Bell Atlantic and NYNEX became part of Verizon, and USWest became part of Qwest.
Probably the only real obstacle to completely re-assembling the old AT&T would be Gov't objections to AT&T and Verizon combining wireless divisions. Even though they use different technologies, it didn't stop Sprint from buying Nextel.