Thank's david 

in my kamailio configuration I am using listen=udp:192.168.131.69:5060 advertise 221.20.20.12:5060 (as mentioned by you)

and when the kamailio sends an invite to sip-gateway I am seeing this 

sip-gateway ip is  192.168.132.67    and this gateway don't have any public-ip assigned ..

Via: SIP/2.0/UDP 221.20.20.12:5060;branch=z9hG4bKaa6a.14c65f5675077ae7b0df70b02516979b.0
Via: SIP/2.0/UDP 192.168.113.112 :50386;received= 192.168.113.112 ;branch=z9hG4bK-d87543-550b984016109617-1--d87543-;rport=50386  

I want  private ip instead of public ip in the first via 

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Thanks and regards 

Vinod.M.N