ATT has a pretty good infrastructure for handling phone calls from customers - being that they are a phone company with more than 120 years of experience in doing so (founded 1885).
So it is probably not as expensive as one might think at first blush.
From my experience with their phone support I'll agree that it probably isn't expensive, but only because it seems to be chronically understaffed. Multi hour wait times appear to be the norm when dealing with AT&T.
Something I heard from a senior developer at another well-established phone/cable/ISP: the company will do everything in its power to avoid having staff pick up the phone, including avoiding development of new products or features that might generate support calls. At the scale of a large telco/ISP, having just 10% of users calling up tech support and talking with a rep for a few minutes adds up.
So it is probably not as expensive as one might think at first blush.