if you want to ensure that your kamailio stays on the path of the dialog for 
following requests you probably want to use record-route headers for this. 
This is usally done with the rr module, record_route() function.

To quote from iptel.org:

"[A] Mechanism by which a proxy can inform user agents that it wishes to stay 
on the path of all further messages is called record routing. Such a proxy 
would insert Record-Route header field into SIP messages which contain address 
of the proxy. Messages sent within a dialog will then traverse all SIP proxies 
that put a Record-Route header field into the message.

The recipient of the request receives a set of Record-Route header fields in 
the message. It must mirror all the Record-Route header fields into responses 
because the originator of the request also needs to know the set of proxies."