AT&T vs T-Mobile

I hope to switch to AT&T from T-Mobile when I purchased my iPhone. I wasn't terribly pleased at this, but I had some months to prepare myself. I especially liked T-Mobile because in the several years that I was a customer, I almost never had to call. I never had any major problems. And their customer service was always great. And their customer service was always available. I just called AT&T to ask about a feature. They're closed. Closed?? I could be using my cell phone at any hour of the day, and they CLOSE? Why don't they just shut down cell service after 8pm too?