I've dug up the U.S. Patent held by AT&T that's at the heart of their just-filed Patent infringement lawsuit against Vonage.
The Patent # 6,487,200, is entitled Packet phone system.
Here's the Patent Abstract:
A packet telephone system which employs a packet network that provides virtual circuits. The packet telephone system employs short packets containing compressed speech. The use of the short packets makes possible compression and decompression times and bounded delays in the virtual circuits which are together short enough to permit toll-quality telephone service.
The packet telephone system employs an intelligent network interface unit to interface between the packet network and standard telephone devices. The network interface unit does the speech compression and decompression and also responds to control packets from the packet network. Consequently, many telephone system features can be implemented in the network interface unit instead of in the switches.
The network interface unit may also be used to provide data connections to devices attached to it. The combination of virtual circuits, with bounded delays, short packets, rapid compression and decompression, and intelligent network interface units makes it possible to build a telephone system with fewer and cheaper switches and fewer links for a given volume of traffic than heretofore possible and also permits substantial savings in provisioning and maintaining the system.
The Abstract contains a description of the Patent that sounds overly broad to me. But then again, I am not a Patent attorney. Just perhaps you are not either. But give this a quick read and submit a Comment with your thoughts.