An arrangement comprising: a first terminal coupled to a wide-area packet-based communication network and installed with a mixer client application for performing a voice or video call using a packet-based communication system implemented over the packet-based network; a plurality of second terminals (102b, 102c, 102d) coupled to the first terminal via a local wireless link, each installed with a respective instance of a lightweight sub-client for participating in said call via the mixer client but having reduced functionality relative to the mixer client; wherein the mixer client is configured to establish the call with a remote, third terminal over the packet-based network using said packet-based communication system, based on a user credential identifying the mixer client as an endpoint of the call within the packet-based communication system from the perspective of the third terminal; each of the second terminals (102b, 102c, 102d) is operable to simultaneously generate a respective audio and/or video stream each associated with a different respective near-end user, and each respective sub-client is configured to transmit the respective stream to the mixer client over the local wireless link, to be used in the call established by the mixer client under identification by said user credential; the mixer client is configured to switch between a multi-terminal state of operation in which the call uses the streams received from the sub-clients and a same-terminal state of operation in which the call uses a stream generated by the first terminal without the streams from the sub-clients, and in said multi-terminal state of operation to mix the simultaneous streams received from the sub-clients at the first terminal, to encode the mixed stream at the first terminal, and to transmit the mixed stream to the third terminal as part of said call over the packet-based communication system under identification by said user credential, characterised in that the reduced functionality of the lightweight sub-client comprises using only a reduced compression of the audio and/or video stream sent to the mixer client over the local link, relative to compression used by the mixer client to compress the mixed stream to the third terminal over the packet-based communication system. a first terminal coupled to a wide-area packet-based communication network and installed with a mixer client application for performing a voice or video call using a packet-based communication system implemented over the packet-based network; a plurality of second terminals (102b, 102c, 102d) coupled to the first terminal via a local wireless link, each installed with a respective instance of a lightweight sub-client for participating in said call via the mixer client but having reduced functionality relative to the mixer client; wherein the mixer client is configured to establish the call with a remote, third terminal over the packet-based network using said packet-based communication system, based on a user credential identifying the mixer client as an endpoint of the call within the packet-based communication system from the perspective of the third terminal; each of the second terminals (102b, 102c, 102d) is operable to simultaneously generate a respective audio and/or video stream each associated with a different respective near-end user, and each respective sub-client is configured to transmit the respective stream to the mixer client over the local wireless link, to be used in the call established by the mixer client under identification by said user credential; the mixer client is configured to switch between a multi-terminal state of operation in which the call uses the streams received from the sub-clients and a same-terminal state of operation in which the call uses a stream generated by the first terminal without the streams from the sub-clients, and in said multi-terminal state of operation to mix the simultaneous streams received from the sub-clients at the first terminal, to encode the mixed stream at the first terminal, and to transmit the mixed stream to the third terminal as part of said call over the packet-based communication system under identification by said user credential, characterised in that the reduced functionality of the lightweight sub-client comprises using only a reduced compression of the audio and/or video stream sent to the mixer client over the local link, relative to compression used by the mixer client to compress the mixed stream to the third terminal over the packet-based communication system.