Further information:

In traditional telephony, the phone company delivers telephony services over a wire, or a "trunk," that connects the customer private branch exchange (PBX) to the public switched telephone network (PSTN).

In New Zealand this service is an ISDN circuit known as a BRA (Basic Rate Access) or PRA (Primary Rate Access). These physical trunks carry the phone calls from the customer site to the PSTN providing the business with phone service. A SIP trunk allows companies to replace separate physical cables with telephone services over a data network -- whether it's through a dedicated line or a shared connection with a data service.

A SIP trunk can deliver much more value to a company than a traditional PSTN trunk. First, there's no real limit to the number of voice sessions that can be carried over a SIP trunk (other than bandwidth), whereas a traditional PSTN trunk is limited to the number of channels available (2 on a BRA, 30 on a PRA). Also, SIP has the abililty to deliver many of the unified communications (UC) functions that companies are looking at today. Chat services, presence, conferencing capabilities, the ability to share applications, etc., can all be delivered over a SIP trunk, allowing companies to look at purchasing UC as a service rather than investing in premise-based hardware and software.

SIP trunking provides more flexible, lower cost telephone services while providing a platform for the future.