While the NSA had the support of all US telcos with its spying efforts, it's come to light that none offered the level of assistance that AT&T did. Recent documents that are part of the ongoing Snowden leaks show the NSA heaping a bit of praise on its relationship with AT&T, saying it was "highly collaborative" and that the company had an "extreme willingness to help."
Beginning in 2003 and leading up to the time Edward Snowden blew the doors off the far-reaching spy efforts, AT&T gave the NSA access to an enormous amount of information through many methods under different legal rules. In particular, AT&T saw no issue with supplying "billions" of emails to the NSA as they passed through domestic servers.
NSA Headquarters in Fort Meade, Maryland
Admittedly, none of this is likely to come as much of a surprise, as we've known for a while the fact that an enormous amount of data was provided to the NSA via these telcos. But the length AT&T went to makes it stand out. It didn't just cater to the NSA when requested for information; it even went as far as offering technical assistance when it was needed.
Not surprisingly, the NSA's financial support of AT&T was much higher than the others - according to the documents, this particular partnership had more than double the funding of the next-largest similar program. AT&T likewise installed many more surveillance points around the country than its competitors, and was the first to use the NSA's newest spying technologies.
It's unclear to what extent AT&T is helping the NSA out today, and unless more future leaks happen, we might never know. AT&T's go-to is "We don't comment on matters of national security". How convenient.