Abstract: In a call and connection separated network that includes a number of domains, the network has a layered structure that includes a connection layer and a service layer. In each connection domain there is a connection control process for controlling hardware. An external network is used to carry a transmission channel that originates from or terminates in the call and connection separated network. Such calls are referred to as inhomogeneous calls. A connection control process at the originating side of an inhomogeneous call uses a routing number to route the transmission channel through the external network. The routing number is administered by a service used by the control process at the terminating point of the call. The routing number follows the signalling protocol of the external network.