Please take a look at the ocs or lync deployment guides. You will see that clients behind the edge server are currently not supported.
Though I can say from our deployment that it can work.
The media handling is the biggest challenge as Ms-ice (used for Nat traversal) i
Could you specify more details, like
how the call fails (like it rings but it does not get established, no media, no ringing at all, .....)
which versions you use
With the x6 calls from the outside might fail due to non authenticated calls which are not forked by the cpl