There have been only two people named Bell that have had a company named after them, or at least major parts of that company historic to that. The first is Glen Bell, who founded the Taco Bell chain of restaurants in 1962 before selling out to PepsiCo in 1978. The second was Alexander Graham Bell, who invented the telephone before creating the American Telephone & Telegraph Company in 1885, creating an intricate system of national telephone companies, the Bell System, largely dependent on manpower (telephone exchanges required large machinery and people to operate them). However, this grew into a company so large that the American government felt that AT&T's grip on the telecommunications network was anti-competitive, and eventually AT&T made a deal with the government, that it would divest the local operating companies into fully independent companies that shared the "Bell System" name (the "Baby Bells").
Granted, the goal of the breakup was ultimately achieved, Bell Atlantic would merge with one of the few reasonably sized indepedent companies, GTE, to form Verizon. Other companies were also formed, like Sprint, originating out of Southern Pacific Railroad (Sprint)'s system that created its own system out of excess right of way from railroads, that would ultimately come to challenge AT&T. But for Southwestern Bell, the company became big. Very big. Southwestern Bell did not waste time to enter the fledgling cellular phone business and by 1987 was already the third largest in the country, and over time expanded, changing names to SBC Communications, and bought Pacific Telesis (and nearly bought AT&T back in 1997, had the FCC turned them down).
The AT&T name finally officially returned to the company when SBC Communications purchased AT&T Corporation, and took their name, changing their name again to AT&T Inc. in 2005, and in 2018, purchased TimeWarner for over $100 billion, adding a massive media empire to their massive telecommunications network. To illustrate just how big the modern AT&T is, I've made this chart. It does not include divested brands--it includes Cingular, but it turns out the actual trademark now belongs to Dormitus Brands. Other brands sold entirely include Atari (the ORIGINAL Atari Inc., not the rebranded Infogrames), Six Flags (like the late AstroWorld featured in the same update this page was put up), AOL (which TimeWarner dropped it from their name BEFORE they sold it to Verizon), or Time (which TimeWarner dropped it from their name AFTER they spun it off, and only changed after AT&T bought the company).
That being said, we officially present the AT&T Inc. company chart, current as of when I last updated it, October 3, 2018, so this wouldn't include any additional buyouts since then, nor any spin-offs or sales, like if they're forced to divest Turner Broadcasting System. Click on it to see the full version.