[Freeswitch-users] Un-authenticated downstream users

I need to achieve a setup where a VoIP gateway box is downstream of Freeswitch but will not authenticate, i.e. calls (INVITES) from this box ( a fixed IP address ) to port 5060 on FS will get routed to the upstream side (I have an upstream gateway defined in sip_profiles), and also all calls from the upstream side will get routed back to the downstream gateway box. This gateway box does not have the configuration to REGISTER, but will simply fling out an INVITE whenever a call is initiated. I don't need to specify specific phone numbers in the FS dialplan (unless this is required by FS). Can anyone suggest a good way to achieve this in the Freeswitch dialplan? Regards, Martyn