Mark writes:

I am unaware of any master clock in AT&T used for sync. I do know that it's quite common for the clock in each central office to run at a significantly different rate.

I just got a reply back from a friend who works for AT&T in Denver. He says:

I'm not too sure about how central offices are set up for clock timing. My past work in AT&T was with the long-distance calling switches (toll offices). Back when AT&T was a monopoly, all the toll switching centers kept their internal clocks in sync via a high-precision clock located somewhere in Missouri. Presumably (but I'm not sure), the toll centers passed along clock sync information to any connected tandem or local central offices. With the breakup, all of this stuff must have been changed dramatically.

Bob Devine