A telephone session is an audio session that is created using either a server-based endpoint instance or a proxy endpoint instance. The proxy endpoint is used when the local user is making or receiving calls using a PBX telephone handset. When the local user has a USB telephone such as a Unified Communications-enabled telephone or just a laptop microphone, the server-based endpoint instance is used. Whether using a proxy endpoint instance or server-based endpoint instance, the audio session used to interact with a remote user is primarily the same. The real difference between a proxy endpoint-based session and a server endpoint-based session is that you cannot add video to the proxy endpoint session. In addition, your organization PBX may not support all of the call control functionality available to a server-based endpoint session.