While it was about 5 years ago, I found AT&T to be the worst in terms of customer service. After signing up for their various services and frequent flyer miles, I had to call:
1) to get the calling cards I had requested.
2) upon receipt of one calling card, to get the second one
3) to let them know that the rates on my first bill were not the ones I had agreed to when I signed up. AT&T said they didn't know that I wanted the lower rates to start immediately and had planned on starting them with my first full month. How pathetic is that?
4) to get my frequent flyer miles.
5) to get my frequent flyer miles (again)
6) to get my frequent flyer miles (again)
I decided that the amount of time I was spending calling them wasn't worth the $5 or so a month I was saving so I switched back to MCI and will NEVER, EVER consider AT&T again!