Setting up SIP client questions (on Mac OS X)

OK, I thought try out SIP-based telephony with my Mac (running MacOS X Tiger). The Mac is in a LAN using RFC 1918 addresses and a Linux PC (running Linux 2.4 and iptables) as a NAT (actually PAT) gateway with an ADSL uplink and dynamic public IP.

As a start, I first created an account on Free World Dialup. I went then looking for appropriate apps. I found three that I downloaded:

- FreeWorldDialup-kphone-mac: seems to lauch, but then nothing happens. Maybe incompatible with Tiger?

- SJphoneMac-277a: starts up OK. I imported the Linux/MacOS X setting file for FWD provided and added my FWD account number and password. Seems pretty easy, but unfortunately it is not transparent. SJphone seems to do STUN to discover my public IP and uses that one for SIP. I was able to successfully call 613 (echo test).

- X-Lite 2.0 release 1103m build stamp 14266: nice interface, seems to work OK. Seems to use STUN as well. I made use of

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configure the phone for FWD. Again, the 613 test worked nicely (at least with the G.711 and the GSM codecs, the other two ones did not yield any usable quality). What I do not understand about the settings is Out Bound Proxy fwdnat.pulver.com:5082 What is this doing? it seems to be necessary though; leaving it blank I do not complete a connection. I thought that an outbound proxy is something in my local network, e.g. a SIP proxy on my local gateway?

Also, even though I can make outbound calls, can others reach me? My NAT/firewall iwould not forward any incoming UDP packets unless there is already a connection established from within my LAN. Or do the SIP clients "keep the line open" to the SIP proxy by establishing such a connection? To receive incoming calls, would I have to use UDP port forwarding (for the SIP port and the RTP port used) with my NAT?

I would appreciate if someone could shed some light into that issue. Further hints and suggestions are welcome. Thanks.