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3CX Phone System for Windows System for Windows
Page 11
How an IP Phone system works
A VOIP Phone System, also referred to as an IP PBX, consists of one or more SIP
standard based phones, an IP PBX server and optionally a VOIP Gateway. The IP PBX
server is similar to a proxy server: SIP clients, being either soft phones or hardware
based phones, register with the IP PBX server, and when they wish to make a call they
ask the IP PBX to establish the connection. The IP PBX has a directory of all
phones/users and their corresponding sip address and thus is able to connect an internal
call or route an external call via either a VOIP gateway or a VOIP service provider.
Figure 1 - VOIP Phone System Overview
The image illustrates how an IP PBX integrates on the network and how it uses the PSTN
or Internet to connect calls.
For more information about how an IP PBX works, visit the IP PBX FAQ at
SIP phones
A VOIP phone system requires the use of SIP phones. These phones are based on the
Session Initiation Protocol (SIP), an industry standard to which all modern IP PBXs
adhere. The SIP protocol defines how calls should be established and is specified in RFC
3261. Because of SIP, it is possible to mix and match IP PBX software, phones and
gateways. This protects your investment in the phone hardware. SIP phones are
available in several versions/types: