A google search (June 2009) on a ‘SIP Trunking’ returns ~500k references.
From Wikipedia (replace ‘connection’ with Trunk)
A SIP (Session Initiation Protocol) connection is a service offered by many ITSP (Internet Telephony Service Providers) that connects a company’s PBX to the existing telephone system infrastructure (PSTN) via Internet using the SIP VoIP standard.
This probably is the most common understanding of SIP Trunking – that being, a SIP-based interconnection from a SIP-based IP-PBX to a SIP Service Provider which facilitate communications to and from the PSTN. Within this context, there’s ample evidence to the value of SIP Trunking. The first, and probably most obvious advantage is a reduction of costs by leveraging IP as the mode of communications between the Enterprise and the Service Provider. Historically, businesses had two distinct and separate network infrastructures – one for data, and another for voice. Maintaining one is easier (but a little bit more complicated at times) than two. If done correctly, it’s definitely cheaper. This point pertains to the LAN as well as the WAN.
Now, if the PSTN were the sole interconnection of interest for SIP Trunking, owning and managing SIP Trunking from the Service Provider POV would be easier. (not easy, but easier) BUT, here things are changing – as more and more businesses (Enterprises and/or Carriers) Transition to SIP, the burden of ensuring that all the SIP Trunks interact successfully with one another falls on the Service Provider. All SIP networks are not built the same. There are numerous differences between Enterprises and Services Providers alike.
Possibly this analogy will help with the understanding. Say each SIP-based network, represents a different language, French, Chinese, Spanish, etc. In order for everyone to communicate with one another, everyone would need to be able to speak and understand everyone else’s native language. There must be a common denominator. Without a common language, then translation services are necessary to contract out to facilitate the communications. The PSTN has historically served as the “common denominator” in the world of IP Telephony integration. The problem stems from the cost of using the PSTN in this manner, both tangible and intangible. First the tangible costs. It’s more expensive to use the PSTN for this reason. Secondly, things get lost in translation. If the PSTN is used as the common denominator, then features other than standard voice (video, HD voice) are not possible, and there’s oftentimes service degradation from multiple encoding/decoding of the voice.
The SIP Trunking Service Providers (at least the good ones) ensure that the necessary translation services required to enable communications do NOT limit the capabilities of SIP, and still enable the end-users to realize the savings of SIP Trunking. This is the true value of the SIP Trunking Service Provider. The Enterprise only needs to ensure that the communications interconnection (SIP Trunk) between them and the SIP Trunking Service Provider are compatible, and the rest can be assumed. This is something that is too often overlooked in the world of SIP Trunking.
Another little know fact is that with international calls, many of the carriers are already using SIP, even if the caller is not. If you start with SIP from your network , you eliminate one of the encoding/decoding steps and end up with better voice quality.
Remember, I am talking about SIP trunks using an MPLS network…NOT the internet. This eliminates the lack of quality control that come to mind for most people when they think of SIP (think Vonage).