I have a cloud server, when I make call from a sip phone (registered with kamailio),

in the INVITE header I am seeing the via header with public IP, is there any way that I can modify via header to send only the private IP.

Please let me know, is it possible to replace.

the via headers that kamailio is sending to sip-gateway is 

sip-phone ip:192.168.113.112

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

how to modify the first via header to have private-IP instead of public-IP..

I tried all the replace functions but it is not working.