Rocketman - callme is working for me, but only after I disabled ports 80 and 443 in the Skype -> Tools -> Settings connection tab, and had my router (Linksys WRT54GS) port forward both TCP and UDP to port 36855 on my N800 (fixed IP address). After that, callme works (the test service instantly calls me back) and echoes my chats.

Note - in my home network, port 80 is already forwarded to a Linux Apache server and port 443 is not being forwarded anywhere so it makes sense for me to disable these ports within Skype on the N800, leaving me with only one option to forward a specific port to the N800 if I wish to receive incoming calls and chats.