Placing Lync 2013 Outbound Calls to a SIP gateway

Hello all. I'm in the process of setting up a simple Lync 2013 system as a small PBX replacement. We have an external SIP trunk provider that serves as our interface to the PSTN. This is a known good trunk/PSTN gateway as Lync can place and receive outside calls with the mediation server's interface exposed directly to the Internet. However, I would like to hide the mediation server behind an F5 BIG-IP LTM for security reasons. I was able to download and install the most recent Lync iApp, and this took care of inbound calls to the mediation server without any issues. However, I can't seem to place outbound calls; I get SIP 503 Service Unavailable errors when I try using a Virtual Server facing the inside interface, with the external PSTN gateway set as a TCP pool member on port 5060. Setting the SIP profile makes no difference, and neither does the network translation setting.

Is there anyone familiar with this kind of scenario for Lync 2013? I can provide more details if someone here understands what I'm trying to accomplish. Much appreciated.

Gerard