SWBT has been the buyer of all of the other bells is a merged with... that is why SBC (Southwestern Bell Corp) emerged as the name even here in California... In the new merger, they chose to go with the off ma bell name of ATT... SBC was clearly the prominent and larger corporation... the guys running SBC are the guys running the New ATT..
SBC stood for SBC. It didn't stand for Southwestern Bell Corporation. It was felt a One Bell Center that Southwestern Bell was too much of a regional name. SBC was chosen to give the corporation a name with a national reach. In fact, One Bell Center's name was changed to SBC Center.
I believe the full name was SBC Communications Corporation. At least it did when I retired from the Southwestern Bell Telephone subsidiary.