I become a bit lazy after using AVM FritzBoxes all the time and god, they surely make all the magic happen by themself
Anyhow I hope somebody has a bright idea for my problem: I have RB493G and it is really a great tool for me.
In my setup it is the NAT router in front of a cable modem for my little home network.
It does DHCP and all the rest, but nothing really fancy.
I made two traffic queues, one for P2P and one for the rest. Every port does get routed through,
I also added port 5060 and 10000 (my providers stun server) to the sip helper.
The VoIP calls does get in the high priority queue btw, so I would assume that is okay.
When I setup the call with my sip telephone everything is fine,
I can talk for minutes and nothing interferes at all.
If I get a call to my SIP telephone it works for a few seconds and then gets dropped.
Talking is possibe in both directions. In the log of the phone I can see,
that it does use the STUN server, but sadly it does not tell my why the call gets dropped.
I assume for some reason my routerboard closes some connection,
maybe somebody got a hint? I have read a bit about session tracking time out,
but not sure if that is it ...
Thanks for your help!