Without Alternate Routing Enabled

If you do not enable H.323 alternate routing, the Oracle® Enterprise Session Border Controller tries one possible next hop gateway when routing H.323 calls even if the applicable local policy has multiple next hops configured. If that next hop gateway fails (either because it is busy or out of service), the Oracle® Enterprise Session Border Controller relays the failure back to the caller, who hears a busy tone.

In addition, the call will only be routed to the other available next hops if the first one is:

A session agent that has gone out of service because it constraints have been exceeded
A session agent that has gone out of service because it failed to respond to a Oracle® Enterprise Session Border Controller Setup request
A session agent group