This post is about how to set up a Lync Server Mediation server to support Direct SIP whilst utilising Gamma Telecom SIP and a Sonicwall Firewall.
This is the way that I did it, you may choose to do it differently. Leave a comment if you got this working another way.
The reason why you need to configure 2 NICs on the Mediation server is because Gamma require the external IP address to be present in the SIP OPTIONS. Normally this isn’t a problem as many firewalls have a SIP ALG. In simplified terms this means that they NAT the SIP traffic so that it appears to come from your external IP address. Sonicwall can do this for SIP over UDP but not for SIP over TCP. Because of this and the requirements from Gamma it’s necessary to have an external public IP address on your mediation server.
There’s a great post here about how to setup Lync Mediation server with “Duel homed” NICs. You’ll need to do this, and as it’s such a good post I’m not going to re-create it.
You’ll also need to stick one leg of your Mediation server in a DMZ on the Sonicwall and configure the DMZ in Transparent mode. Another good post here detailing how to setup your Sonicwall DMZ to support this.
Here’s how your mediation server should look in the topology builder.
Here’s the Gateway.
And here’s the Trunk.
That should get you going.
Bye for now.
The Geek in Disguise 
Thanks for the article! We are having an issue with our setup (Direct connect to Gamma SIP trunk) whereby any incoming call will a blocked caller id fails…….do you have any idea why this might be – starting to drive me mad!
Hi Lee, you could try changing the FQDN of the PSTN Gateway in the Topology Builder from its name to its IP address.
Hi James Thanks for this post. I have setup the topology, do i need to add any configuration to the Lync server? If no then how do I test?
Thanks for your help.