Hi Jean,
as pointed out earlier, the ACK is probably broken. Every packet after the
200 OK should have the contact of the 200 OK in the Request URI. Your
example packet carries probably the original request URI, which is most
certainly wrong, exept if you use some topology hiding.
This is something you can't fix but the voip provider has to fix. They
apparently can't handle proxied SIP connections.
Regards,
Sebastian