[jitsi-users] jitsi on FritzBox via VPN

I have a strange problem on using jitsi SIP with a FritzBox. The connection to the FritzBox SIP server is made through a VPN application.

My local IP is, my local IP of the VPN tunnel is The IP of the FritzBox is
After entering the SIP account informations I can successfully register to the FritzBox SIP server. But when I start a call the SDP of the INVITE directs the RTP stream to instead of Of course the FritzBox does not know about and so voice fails.

Is there a way to define the local IP jitsi should work with? Or is there another solution to that problem?

BTW: The VPN application is not creating a extra network interface. So 'ipconfig |find ""' on a windows machine does not show anything.