Lync 2013 mediation server with Primary and PSTN IP - NIC binding order is important

When you use a separate IP on your mediation server for SIP PSTN gateway, make sure that clients communicate with primary IP and not with PSTN IP. Verify NIC binding order to see which interface is binded first (primary must be first) - binding can change when you do V2V as example. Change binding order:  http://support.microsoft.com/kb/894564

Remove default gateway from PSTN IP, use static routing to your SIP gateway