look up Texas DSP VOIP in google and you'll find nearly every VOIP product using VOIP. The idea that by sticking the right parts together will result in a usable product doesnt deal with the user/administrator interface. A lot of software is needed to get these working, logic analysers to see where things go wrong, protocol sniffers to monitor traffic and maybe a VOIP server/router. The key to good transmission & reception is to do with the delays between packets, too long and blahhhhh. For an ultra simple setup VOIP router or provider is required, just use the IP of the other phone. This is usually turned off on most systems as a lot of phones are attached to DHCP based systems.