Finally I have got a SIP softphone working through my firewall.
I spent hours and hours trying different inspection rules and nat traversal stuff and eventually gave up, but after waking up obscenely early this morning I decided to give it a quick googling.
Five minutes later it was working - seems all I had to do was disable the SIP ALG on my Cisco 837 router:
no ip nat service sip udp port 5060And it works !
Of course now I find that sound doesnt work in Ekiga (grumble, grumble), but Twinkle works perfectly.