Having trouble with STUN server

I have a R200 and am having problems getting this to work with our STUN server.  The R200 is behind a Canopy SU, and using the DMZ'd IP address through the Canopy's NAT.  When a call connects, there is nothing but silence.  This is the same behavior I see when using a Grandstream or Linksys ATA if the STUN server settings are not correct.  The R200 has NAT Traversal set to STUN, with the correct STUN server and port number.  NAT Refresh interval was set to 20, but I noticed that the only time I would see packets going to our STUN server would be when a call was placed.  No matter what I change the NAT refresh interval to, it doesn't seem to actually send the packets.  I do see these refresh packets when I'm using one of the other branded ATA's.  Any suggestions?  This is using firmware version 4.2.2-R1.

I have an update.  I was able to get this to work by Enabling SRTP on the R200, and then enabling encryption on the corresponding extension on the PBX.  I'm not sure why this would need to be done though, as it is not necessary when using our other ATA's.  I'm still curious as to why the NAT Refresh doesn't seem to be working though?  Is this a known issue?