I just learned that AT&T's CallVantage is not using SIP, and are using MGCP, but that SIP is on the edge is on their roadmap, and all of their core elements use SIP in their network.
Also, AT&T's CallVantage offers no QoS (today) but their enterprise grade offerings have various Service Level Agreements (SLA's). As far as applications go, everything has been developed in house, no third party applications server, which is smart. No royalties, no licensing. I hear Webley has taken a similar approach.VoIP Watch
Build your own application server? This sounds crazy to me, but it also means that AT&T has been planning this for a long time. Will have to have a peek to see if I can find out what companies like VocalData are charging: maybe AT&T went this route because they are paying per-subscriber licensing fees?