The traditional public switched telephone network has a strictly hierarchic architecture and a star structure. The individual subscriber lines are connected to a local exchange, which in turn communicates with trunk exchanges, main and central exchanges. Lines within a local exchange typically have the same area code. To call a number outside the local exchange the area code must be added. International calls require dialling the country code. These start with a double zero.
Since the introduction of Voice over IP telephony (VoIP), traditional public switched telephone networks are slowly being replaced. Voice over IP uses the internet for speech communication and transmits the voice data as individual data packets. Unlike public switched telephone networks, there is no static connection between the two subscribers, instead data is dynamically routed over the data network using a virtual connection. Since each data packet has a destination address, the packets can even be routed differently over the network. On the other end they are reassembled in the correct order and converted back into analogue speech.