I would assume the you put the VPN phone in a residential network as a teleworker. the first thing I would do is to stop downloading while on the phone, as without QOS on the home network, any link saturation result in robotic voice. If you can't put QOS and you need to be able to make internet transfers while on a call, the other option would be to get a faster internet connection in order to keep the link from ever saturating (upload and download). You could also make sure you are using a WAN tolerant codec like G711 and iLBC. You could also make sure the same thing doesn't happen on the office side.