System phone lines are dedicated to act in a particular communication system of the manufacturer. Each manufacturer of communication systems uses their own signaling protocols, which enables the operation of a dedicated system phone line.
IP SIP phone allows for making and receiving calls through VoIP technology using SIP (Session Initiation Protocol). It is currently the dominant signaling protocol used in VoIP. IP SIP phones work with many types of VoIP platforms and have no restrictions as far as the type of dedicated communication system is concerned, as is in the case of system line phones.