Set this on your sofia profile <param name="manual-redirect" value="true"/>

You can set these variables sip_redirect_profile, sip_redirect_context, sip_redirect_dialplan, 

When a redirect happens you get these variables - sip_redirect_contact_%d, sip_redirected_to, sip_redirect_contact_user_%d, sip_redirect_contact_host_%d, sip_redirect_contact_params_%d, sip_redirect_dialstring_%d, sip_redirect_dialstring, sip_redirected_by

Then its up to you to process the redirect in your dialplan, If you don't set the sip_redirect_context then it'll default to redirected context and XML as the dialplan.

/b