[Serusers] why would a client behave differently for nat on outbound as opposed to inbound

Couldnt think of a better subject, basically I have a handytone (HT) , and a xlite behind the same router, which is natting. SER is external Now when I call HT to xlite I hear absolutely nothing in either direction, and nuthing except for a request from xlite hits the mediaproxy...but when xlite calls HT, mediaproxy lights up like a xmas tree, and all is well. What I can see is that if the router is the same one, then surely something in the HT settings (I have no NAT traversal set in HT) causing this, or something in xlite telling it what to do, I just cant see what. Oh xlite/HT to pstn works fine, and inbound from pstn to HT/xlite is also fine voice both ways. Iqbal