A near-end user terminal for conducting a call with a far-end user terminal between a near-end user being a user of the near-end user terminal and a far-end user being a user of the far-end user terminal, wherein the far-end user terminal is connected to a packet-switched network, the near-end user terminal comprising: at least one first network interface operable to form a connection between the near-end user terminal and a packet-switched network, for conducting the call over the packet-switched network using a packet-based voice calling service; a second network interface operable to form a connection between the near-end user terminal and a PSTN network; memory comprising one or more memory units, the memory storing a communication client application; and processing apparatus comprising one or more processing units, the processing apparatus being arranged to run the communication client application; wherein the communication client application is configured so as when run on the processing apparatus to perform operations of: initially establishing the call with the far-end user terminal all the way over the packet-switched network by using the first network interface to conduct the call via the connection between the near-end user terminal and the packet-switched network and by using said connection between the packet-switched network and the far-end user terminal; detecting a failure of the connection between the near-end user terminal and the packet-switched network; and in response to detecting the failure, during the call, switching the call from being conducted all the way over the packet-switched network by using the first network interface to being conducted at least part way over the PSTN network by using the second network interface to conduct the call via the connection between the near-end user terminal and the PSTN network, wherein the communication client application is configured such that said conducting of the call at least part way over the PSTN network comprises: conducting the call only part way over the PSTN network, by conducting the call via a gateway between the PSTN network and the packet-switched network using a gateway PSTN number mapped to a user ID identifying the far-end user within the packet-switched voice calling service, and via a connection between the packet-switched network and the far-end user terminal via the packet-switched network using the packet-switched voice calling service and the user ID of the far-end user. at least one first network interface operable to form a connection between the near-end user terminal and a packet-switched network, for conducting the call over the packet-switched network using a packet-based voice calling service; a second network interface operable to form a connection between the near-end user terminal and a PSTN network; memory comprising one or more memory units, the memory storing a communication client application; and processing apparatus comprising one or more processing units, the processing apparatus being arranged to run the communication client application; wherein the communication client application is configured so as when run on the processing apparatus to perform operations of: initially establishing the call with the far-end user terminal all the way over the packet-switched network by using the first network interface to conduct the call via the connection between the near-end user terminal and the packet-switched network and by using said connection between the packet-switched network and the far-end user terminal; detecting a failure of the connection between the near-end user terminal and the packet-switched network; and in response to detecting the failure, during the call, switching the call from being conducted all the way over the packet-switched network by using the first network interface to being conducted at least part way over the PSTN network by using the second network interface to conduct the call via the connection between the near-end user terminal and the PSTN network, wherein the communication client application is configured such that said conducting of the call at least part way over the PSTN network comprises: initially establishing the call with the far-end user terminal all the way over the packet-switched network by using the first network interface to conduct the call via the connection between the near-end user terminal and the packet-switched network and by using said connection between the packet-switched network and the far-end user terminal; detecting a failure of the connection between the near-end user terminal and the packet-switched network; and in response to detecting the failure, during the call, switching the call from being conducted all the way over the packet-switched network by using the first network interface to being conducted at least part way over the PSTN network by using the second network interface to conduct the call via the connection between the near-end user terminal and the PSTN network, wherein the communication client application is configured such that said conducting of the call at least part way over the PSTN network comprises: conducting the call only part way over the PSTN network, by conducting the call via a gateway between the PSTN network and the packet-switched network using a gateway PSTN number mapped to a user ID identifying the far-end user within the packet-switched voice calling service, and via a connection between the packet-switched network and the far-end user terminal via the packet-switched network using the packet-switched voice calling service and the user ID of the far-end user.