The integration of SIP telephones and VoIP routers into existing telephone structures means that we have to take a fresh look at familiar functions such as forwarding calls. Call forwarding means that a call that has already been placed (routed) is redirected to a new destination either spontaneously by the user (connect call) or by automatic call forwarding set up in advance. SIP-based VoIP telephony uses processes which are fundamentally different to previous technologies. For example, ISDN and analog terminal devices require a telephone exchange that usually has to continue to manage the connection after forwarding. SIP telephones can forward calls without any need of a telephone exchange: The devices make a connection over the shortest possible route and the call router stops its management function immediately after the connection has been established. The SIP exchange is also able to handle signaling over SIP and the actual data transfer over RTP in different ways.
Due to the differences arising from the various types of terminal device, the easiest way to understand call forwarding in a LANCOM VoIP router is to consider different scenarios and to explain the terminology.
