Sipp & call receiving/echoing

I’m using SIPp to generate calls from a dedicated server (UAC scenario) to another dedicated server running Asterisk 10.
I have started a third server in order to receive/echo the calls. I’ve configured a “receiver” scenario and started SIPp as UAS on this server.

The “SIPp UAC server” is correctly reaching the “Asterisk server”, but I wonder how to redirect the calls to the “SIPp UAS server”.
Could you please advise ?

Flow: UAC ----> ASTERISK --?–> UAS