[Serusers] SER + mediaproxy or STUN

Hi I have installed ser with STUN support, since my routers are not symmetric, in theory this should solve natting issues, however I seem to get completely random results. I am using xlite softphones behind the SAME nat to make a call, they can call out to pstn (via asterisk), however one phone can call the other across the internal network, but not vice versa, even though both PC's are on the same network. The ser/asterisk/stun boxes are on apublic network. I came across a post which using a tcpdump would draw a diagram of the sip call path, does anyone have the url/name for that, I just cant see what wrong in the sip debug messages, maybe a pic will help me out. tks Iqbal