To enable cross organization/premise Free/Busy lookup for users located on Exchange 2007 servers when there's an organization relationship in place with the business instance of Microsoft Federation Gateway it's necessary to add availability address space for remote organization and have at least one Exchange Server 2010 SP1 CAS. This's to provide Ex2007 server with a valid path to route remote availability requests. Because there's only an Organization Relationship in place between the organizations then there's no way for Ex2007 servers to utilize this connection without proper configuration.
"The solution is to run Add-AvailabilityAddressSpace CMDlet and the proxyUrl parameter to allow the relay to occur from Ex2007 to Ex2010 sp1 servers. The ProxyUrl parameter included with Exchange 2007 SP2 specifies whether to direct a Ex2007 Client Access server to proxy its free/busy requests through an Ex2010 sp1 CAS when requesting federated free/busy data for a user in another organization for which you've an organization relationship in place. The proxyUrl should point to local Ex2010 servers availability service (EWS URL)."
After this's configured correctly, the flow of information will go as follows: