The 200 OK to Fongo / Fibernetics does contain your correct public IP in the contact header, but you aren’t receiving their ACKs and are re-transmitting the 200 until it times out. We know that they got the 200, otherwise they wouldn’t know where to send the audio you are hearing.
My guess is that the ACKs are coming from a different public IP (because of the multiple VIAs and Record-Route headers) and your router is blocking them. Can you capture traffic on the WAN interface of the router and see whether the ACKs are present? If so, forwarding UDP port 5060 from their IP addresses to 192.168.192.8 should be a fix.
If the capture is difficult, try forwarding UDP port 5060 from 208.65.240.0/21 to 192.168.192.8 port 5060.
http://whois.domaintools.com/208.65.240.44