Set the "Reorder Route List" CallManager service parameter to True (defaults to False). That will allow CallManager to do what you want, which is round-robin among devices in a route group that have equal priorities. You reach this from CCMAdmin through the Service menu, Service Parameters, and then pick CallManager out of the services list.
The help text for that service parameter, as an FYI:
"This parameter determines whether the first route group in a route list, whose devices/gateways have the same device selection order, will receive calls in round-robin fashion. Valid values are True (route calls in round-robin fashion for devices that have the same device order selection) or False (route calls in a linear fashion). Cisco CallManager arbitrarily chooses which device starts the round-robin routing. This parameter is only applied at the route group level; it is not port-specific. For example, if gateway1 has the device selection order of 1, gateway2 has the device selection order of 2, and gateway3 has the device selection order of 1, and this parameter is set to True, then calls will route to gateway1 and gateway3 in round-robin fashion (Cisco CallManager arbitrarily decides which of the two devices goes first); if both gateway1 and gateway-3 are busy, then gateway2 will route the call. Because this parameter applies only at the route group level (not device/gateway level), calls are routed to gateways that have ports configured as specified by the Port Selection Order, either Top Down or Bottom Up. For example, the Port Selection Order determines whether gateway4-port1 is chosen before gateway4-port2.
This is a required field.
Default: false."
