A first network comprising: one or more first end-user computer terminals; a gateway to a second, packet-based network comprising one or more second end-user computer terminals installed with a communication client of the second network; one or more telephone units for performing calls with one of the second end-user computer terminals over the second network; wherein each of the one or more first end-user computer terminals is installed with a communication client for performing calls with the second end-user computer terminals over the second network; and characterised in that: the gateway comprises a switching apparatus configured to automatically detect whether a user uses one of said first end-user computer terminals or one of said telephone units to perform a call, in order to select respectively between a first state in which the switching apparatus is configured to route both call set-up signalling and call media to the first end-user computer terminal and a second state in which the switching apparatus is configured to route call set-up signalling to the first end-user terminal but route the call media to the telephone unit; and wherein the switching apparatus is configured, on condition of detecting use of the first end-user computer terminal, to select the first state; and on condition of detecting use of the telephone unit, to select the second state. one or more first end-user computer terminals; a gateway to a second, packet-based network comprising one or more second end-user computer terminals installed with a communication client of the second network; one or more telephone units for performing calls with one of the second end-user computer terminals over the second network; wherein each of the one or more first end-user computer terminals is installed with a communication client for performing calls with the second end-user computer terminals over the second network; and characterised in that: the gateway comprises a switching apparatus configured to automatically detect whether a user uses one of said first end-user computer terminals or one of said telephone units to perform a call, in order to select respectively between a first state in which the switching apparatus is configured to route both call set-up signalling and call media to the first end-user computer terminal and a second state in which the switching apparatus is configured to route call set-up signalling to the first end-user terminal but route the call media to the telephone unit; and wherein the switching apparatus is configured, on condition of detecting use of the first end-user computer terminal, to select the first state; and on condition of detecting use of the telephone unit, to select the second state. the gateway comprises a switching apparatus configured to automatically detect whether a user uses one of said first end-user computer terminals or one of said telephone units to perform a call, in order to select respectively between a first state in which the switching apparatus is configured to route both call set-up signalling and call media to the first end-user computer terminal and a second state in which the switching apparatus is configured to route call set-up signalling to the first end-user terminal but route the call media to the telephone unit; and wherein the switching apparatus is configured, on condition of detecting use of the first end-user computer terminal, to select the first state; and on condition of detecting use of the telephone unit, to select the second state.