AT&T has long provided a sub-par Internet connection service.
Their wireless router and DSL modem "U-Verse" gateway device lacks a true bridge mode, has a small NAT table, and you must use it because it contains a 802.1x certificate. You can use your own router with a rather involved hack: http://blog.0xpebbles.org/Bypassing-At-t-U-verse-hardware-NA... They instructed customers (including small businesses) to get off 10.X.X.X, presumably because they were considering CG-NAT, although they have since done some IPv6 deployment.
The original AT&T faded away with declining long distance revenue which they milked to the end until cellphones and Internet access became the main market. It took tax credits meant for "video dial tone" broadband deployment and used them to build their cellular network.
Its ADSL deployment use PPPoE and ATM which can cause MTU mismatch. ATM itself is a telco mistake. European access oligopoly members wanted 32 bytes because that suited voice frame transport and they thought data transport would never sell. American counterparts wanted 64 bytes for better data delivery. They compromised on 53: 48 for payload and 5 for header. With newer "U-Verse" ASDL they've finally eliminated this layer of overhead. (But no more 3rd party modems).
It decided to deploy VDSL instead of make the true upgrade to fiber. But they also cut costs by not building enough remote terminals so many customers have slow and unreliable links. Fast VDSL speeds (e.g. 100Mbps) aren't possible if customers are a mile away, or more. How much money have they spent on truck rolls diagnosing issues? Is it less than just building a more robust network to begin with?
Since 2005 they've been complaining that customers are using too much Internet and they need to slice and dice the Internet with data caps, application specific throttling, etc. And get more tax credits and deregulation which they promise will be used for broadband investment.