I think bigL was referring to disabling SIP ALG on his router. Not all routers have that, not all that do have a user control to disable it and not all use the term SIP ALG in the management screen.
If your router is a separate item from your modem you can try temporarily connecting the OBi directly to the modem. You will need to reboot the OBi and if you have given it a static IP address, adjust that appropriately. Obviously not a fix as the OBi will be the only device connected to the internet but a test conference or two should give some pointers.