A method and system for providing interworking between a first and a second network of different types, preferably an IMS and a CS (Circuit-Switched) network. A call received by the first network and directed to a terminal unregistered from the first network, is forwarded to the second network, whereas when the call is received by the second network and is directed to a terminal unregistered from the second network, the call is not forwarded to the first network. The same identifier, preferably the same E.164 number, is used in both networks for identifying the called terminal. Preferably, the first network is informed on the registration of the terminal to the second network, the first network using this information as basis for the decision whether or not to route a call to the second network.