SIP and ATA

I found a post, http://hubpages.com/hub/Build-Your-Own-Cheap-VOIP-Phone-Connection and will summarize what I read here. This guy is trying to create an incoming phone line for himself for the cheapest possible way through VOIP. ATA (Analog Transmission Adapters) is a box that converts analog to digital signals and vice versa. "Unlocked" ATA (available at voxilla store, techtoast; cost under $100) hooked with the router and the phone and configured correctly with the SIPPHONE vendor allows the vendor to route calls to the phone connected.
SIPPHONE vendors: SIP (Signal Initiation Protocol) is an application layer protocol that  serves as a link between a phone and the internet and is widely used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP). 
More on SIP coming up...
I feel that to establish your own VOIP system, the first and only thing required will be the SIP server. This is the one that routes calls to the ATA and must be the one that routes calls to the PSTN (Public Switched Telephone Network).