503 Service unavailable received at kamailio SIP server is forwarded as 500 server internal error to the UAC. Which seems fine based on the fact that there is no retry-after header. 

Would like to know if there is any way to to control the SIP Headers like adding Proprietary SIP Headers to the 500 "Server Interrnal Errror" created by kamalio stack for 503 service unavailable (without retry-after Header)

It seems that Kamalio does not retain the proprietary sip headers received in 503  to 500 response forwarded.

Any insight into the issue would be helpful 
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