Setup QOS at the remote sites is recommended but keep in mind that if the bandwidth is in use for a large file transfer before the call is established it will not slow down the transfer because the a phone call came in. It will only add to the amount of bandwidth that is being used. It does not cut back on the current data that is in use by the file transfer, it will however give priority to the voice packets if you are actively involved in a conversation and then you download say your email. It will limit that due to the QOS settings.