the traditional telephon system we used to use based on whats called TDM
while with IP telephony it is based on IP comunication, as any computer has an Ip address the telephon will has too, but the will be call control system that map the phone number to the assigned IP address in Cisco technology the Callmanger or the new version called univief communication manager will be the call control system, and with IP telephony the main carrier for the comunication is the IP/L3 and the cisco IP phon use sccp protocol for call signaling over tcp 2000 with CUCM once the call connected between two phones theough the CUCM the comunications between the two ip phones will be dirct with the need to the callmanager and it will be over UDP with automaticly negociated udp port number
this is in brief
what component you need
you need first a reliable infra stracture such as switches and better to be configureable and support QOS
QOS gives proirity to voice for example over other kind of traffic to garntee the delevery and avoid any delay or latency in real-time traffic delevry
and also you need call control system
in traditional phon systems(non-IP) called PBX
with cisco the new version is CUCM
which is based one a server
however if you have small site with less that 240 user you can use a router to achive the call processing called callmanager expres
also if you have multisite deployment in this case you need routers voice enabled to do a gateways job for you between sites
hope i was helpful in this
rate if helpful
