A hybrid arrangement allows an existing circuit switched telephony network to be deployed on a packet switched network. The circuit switched underlying transport network is replaced with that of an IP based package switched network, while the circuit switched infrastructure, such as terminals, telephone exchanges and the like are retained. Both a subscriber and a local telephone exchange are connected to, and separated by, an IP based packet switched network, such as the Internet. Signalling between the subscriber and exchange are effected using a standard user to network protocol, such as V5.1 for PSTN and ISDN BRI or DSS1 for ISDN PRI. This standard user network is overlaid on an IP based network protocol, such as TCP or UDP on IP.