When the IP phones boot up and acquire their IP addresses from DHCP, option 66 is typically used to indicate to the phone which TFTP server to use -- either of which could be the IP PBX system or VoIP server. The phones then look for files on the TFTP server which provide them with their configuration, which is usually created using an interface on the IP PBX or servers.
Once the phones have their configuration, they register with the IP PBX or servers and phone calls can typically be made at that point.
If there are multiple sites that need to be connected together, generally routers are used to provide layer 3 QoS across the WAN network. Also bandwidth management devices such as a Packeteer are often used.
This was first published in March 2006