For companies wanting to get the most out of their installed IP PBX’s connecting to the traditional PSTN network, SIP Trunking is the solution. Formal definition: A SIP trunk is a service offered by an ITSP to use SIP to set-up communications between an enterprise PBX and Internet Telephony Service Provider.  The trunk can handle as many voice sessions as the enterprise needs.  The IP-enables landscape provides a platform for enhanced communications while reducing the costs of service.
Benefits of SIP Trunking:

• Enhanced communications and productivity
• Elimination of geographic constraints
• Eliminates the needs for traditional fixed PSTN lines
• Easy to modify or expand capabilities
• Eliminates costly Basic Rate Interface (BRI) and Primary Rate Interface (PRI) subscriptions
• Optimal utilization of bandwidth by delivering both data and voice in the same connection
• Redundancy with multiple service providers and links

There are three components necessary to successfully deploy SIP trunks: a PBX with a SIP-enabled trunk side, an enterprise edge device understanding SIP and an Internet telephony or SIP trunking service provider.

PBX
In most cases the PBX is an IP-based PBX, communicating with all endpoints over IP, but it may just as well be a traditional digital or analog PBX. The sole requirement is that an interface for SIP trunking connectivity is available.

The enterprise edge device
The enterprise edge component can either be a firewall with complete support for SIP or an edge device connected to the firewall, handling the traversal of the SIP traffic.