A SIP Trunk is a connection from your telephony solution to the PSTN via a SIP Provider. For Lync, this means the route to the PSTN is via the SIP provider, with all the signaling traffic being SIP and the audio being sent via RTP. This avoids having to have a local gateway to convert the signaling and audio into what the PSTN interface requires. SIP providers can be a cheaper PSTN Interconnection alternative to going direct to your local PSTN (e.g. British Telecom).
This means the PSTN connection is at level 3 in the ISO stack, not level 2 (which the normal PSTN connection uses). Someday, in the not overly far future I suggest that all PSTN connections will be via direct SIP, although it may take a few decades to get there.
Sip Trunks can provide many advantages. They can often be a cheaper PSTN Interconnection alternative to going direct to your local PSTN (e.g. British Telecom). They are also are a lot easier and faster to provision than traditional Level 2 connections. And, in many cases, you can create a short term trunk (e.g. for some event) more cost effectively than a traditional PSTN line.
With Lync 2013, SIP Trunks are very easy to integrate into your environment and can provide great cost savings – if nothing else, you don’t need separate PSTN gateways! If you are planning on deploying Lync Enterprise Voice, SIP trunks are something worth looking at.
Enterprise Connect Webinars are holding a 1-hour webcast on Jan 29th at 19:00 GMT – sadly late in the day for the UK, but mid afternoon EST and late morning on USA’s west coast. This web cast will describe the opportunities and challenges of SIP Trunking and will feature a detailed case study. You can signup for the webinar on line: Here.