Establishing a VoIP Call through an IP PBX
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4/8/2010
An Internet Protocol Private Branch Exchange (IP PBX) acts as both a Session Initiation Protocol (SIP) server and a voice gateway. Several IP PBX devices can be arranged in a hierarchy.
VoIP devices are addressed using both SIP AND TEL URIs.
This means a device is addressed with both a standard phone number (like (425) 555-0101) and a SIP URI (like someone@example.com).
When an outgoing VoIP call is made through an IP PBX using a standard phone number, the IP PBX attempts to match the phone number with a SIP URI. If the IP PBX matches the phone number with a SIP URI, then the caller and the callee can establish a peer-to-peer VoIP call. The devices then establish the peer-to-peer call without using the IP PBX for routing.
If the IP PBX cannot match the phone number with a SIP URI, it attempts to pass the call to its parent IP PBX for routing. If there is no parent available, the call is sent over the Public Switched Telephone Network (PSTN). In that case, the calling device and the IP PBX communicate using VoIP, and the IP PBX and the receiving (called) device communicate over the PSTN.
When an incoming call is routed to an IP PBX over the PSTN, the IP PBX matches the incoming standard phone number with a SIP URI. Once the SIP URI has been found, the IP PBX and the destination device communicate using VoIP.
An IP PBX is not required for incoming peer-to-peer voice calls.
See Also
Concepts
VoIP Technology Overview
Establishing a Peer-to-Peer VoIP Call
VoIP Data Pathways Between Phones