Put one of these together today. Without seeing this page. Well, we arrived at the same solution, was pretty straightforwards.
EXCEPT
We can't transfer calls. The extensions can call each other and all is fine. But when Ext 1 calls Extn 2, then Extn 2 does a transfer to Extn 3 (blind or attended) then once the transfer completes (it's all OK up to that inccluding the 2<->3 voice during attended trfr) theres no audio either way between 1 and 3.
Tried putting the IP of the 3CX box in the RTX server confiig as "outgoing proxy" but that made no diff.