There are some really good resources available on how to setup SIP trunking between Lync and Microsoft Certified gateways (such as Sonus and Audio codes).
One of my favorite resources is from Iain Smith at NorthernLync found here
Problem
Unable to set the port to 5061 when adding a PSTN gateway in Lync Topology builder.
Notice how finish is grayed out and we have an error next to the port.
Cause
The reason we have this is because by default the mediation servers are set to use TLS and thus listening on port 5067 and TCP isn’t enabled. You can check this by expanding the Lync Server 2013 -> Standard EditionEnterprise Edition. Right click on the Front End where the Mediaiation service is co-located and select Edit Propoerties
Scroll down to the Mediation Server details to see the Mediation Server configuration
Solution
Simply enable TCP by checking the check box and set the port range
Returning to the PSTN Gateway folder in Topology builder and selecting New IP/PSTN Gateway you will notice that you are now able to add the gateway as TCP using port 5060
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