SIP trunks can be used for a wide variety of call processing  solutions, some of which are very cost effective as in Free!   We have used  SIP tunks to interconnect ShoreTel systems over the Internet, complete with four digit dialing between systems!    Most recently,  we have been able to create a ShoreTel compatible audio conference server at a fraction of the cost normally associated with these servers.   The most costly part of  this conference solution  was the ShoreTel SIP Trunk Licenses at $50 per port and the Server Hardware.    SIP is a wonderful solution and enables you to interconnect a variety of exciting options to your VoIP solution that are vendor specific yet provide the highest level of interoperability.  We now provide an audio conference server, (you provide the server) free to all of our ShoreTel annual support subscribers.  We will send you a link to download the server, just for letting us quote your support contract renewal!