Make sure you're good at negotiating your starting salary, in the (rare) case of promotion, you'll get maximum 10% increase, even if your new team literally makes 50% more than your old. Oh yeah, don't forget that getting a raise needs approval from VPs, so it takes around 6 months and can be declined at any time.
People tend to be evaluated not by performance or skills, but mostly by length of employment with AT&T...
Depending on your position it may get dull in very short time, usually you stop learning new things after 6 months to 1 year.