[Serusers] Finding IP address of VoIP gateway sending the 5xx

Valued Friends, I am trying to process 5xx response codes and to resend the INVITE to an alternative VoIP gateway upon receiving an 5xx response from a VoIP gateway. So, I need to somehow figure out the IP address of the VoIP gateway from which SER receives the 5xx response. I tested and I receive 503 response codes only in onreply_route and not in failure_route at all. The 503 response does not include any "contact" fields that Would indicate the IP address of the VoIP gateway from which The 503 response is being received at SER. Do you know how I can figure out the IP address of the VoIP gateway from which SER is receiving the 503 response?! Thanks ramin