AT&T was the telephone monopoly in the US until the government broke it up into 9 regional companies in the 70s. These companies have since re-merged into 3 companies: AT&T, Verizon and Lumen Technologies. Lumen is the only one that isn't in the cell phone business. It used to be called CenturyLink until very recently.

Unix and C were invented at AT&T's Bell Labs.

https://en.wikipedia.org/wiki/Regional_Bell_Operating_Compan...