For companies wanting to make full use of their IP-PBXs and not only communicate over IP within the company, but also outside the company a SIP trunk provided by a Service Provider to connect to the traditional PSTN (public switched telephone network) & 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 lines with connectivity via a SIP trunk service provider on the Internet.

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

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

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

The Router – The PBX on the LAN connects to the service provider via the router. The router can either be a firewall with complete support for SIP or router connected to the firewall, handling the traversal of the SIP traffic.

The ITSP – On the Internet, the service provider provides connectivity to the PSTN (Public Switched Telephone Network) for communication with mobile and fixed phones.