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What Is SIP Trunking?

A SIP trunk is a service offered by an ITSP (Internet Telephony Service Provider) that permits businesses that have a PBX installed to use Voice-over-IP (VoIP) to connect to tranditional land lines by using the same connection as the Internet connection (usually broadband).

Organisations wanting to make full use of their installed IP-PBXs and not only communicate over IP within their "private network", but also outside, a SIP trunk provided by an Internet Telephony Service Provider to connect to the traditional PSTN network is the solution. Unlike in traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk allows a company to replace these traditional fixed PSTN lines with PSTN connectivity via a SIP trunking service provider on the Internet.

SIP trunks can offer significant cost-savings for organisations, eliminating the need for local PSTN gateways, costly ISDN BRIs (Basic Rate Interfaces) or PRIs (Primary Rate Interfaces).

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

The 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 Firewall element

The PBX on the LAN connects to the ITSP via the firewall device. This device must have complete support for SIP to handle the traversal of the SIP traffic.

The ITSP

On the Internet, the ITSP (Internet Telephone Service Provider) provides connectivity to the PSTN (Public Switched Telephone Network) for communication with mobile and fixed phones.