AT&T Mobility has basically bought or been bought by or merged with many other companies to take over their customer base, and merged with companies who merged with other companies...

Just an incomplete list of companies that have gone away to AT&T Mobility includes:

Metromedia
McCaw Cellular
Pacific Telesis
PacBell
BellSouth
SBC
Cingular
AT&T Wireless
Cellular One
Ameritech (later sold, now Verizon)
SNET
Comcast Cellular
Dobson Celluar
Edge Wireless
Centennial Wireless
Wayport (wifi hotspot network)

And coming soon: T-mobile

So, they basically became the largest company by buying the competitors (and with them, their customers).

AT&T/Bell Labs/whatever you want to call them, did a lot of great work for UNIX, C, C++ etc. in the old days. Really invaluable work. Some of the most famous names in our field work or worked for the company.

But, for ordinary telephone customers, they've never been very friendly... AT&T had a land-line monopoly until the early 1980's when the US government broke them up. In the "bad old days" you couldn't buy your own phone and use it, you had to lease or buy the phone from AT&T. (sounds familiar...)