Yes, it works great. On the properties of the Mediation server just go to the next hop tab and enter the IP address or FQDN of your ITSP. Right now there are only a small number of officially tested and supported SIP trunk providers (see
here for more info) but it's been found to work with other ITSPs who support TCP SIP connections.
I
posted the info to connect to Global Crossing a while back. It may be helpful when you are speaking to your ITSP. Please post your results, especially if you get it to work with a new provider.
Mike Stacy | Evangelyze Communications | http://www.evangelyze.net/cs/blogs/mike