An IP phone uses Voice over IP technologies allowing telephone calls to be mae over the internet instead of the ordinary PSTN system. The phones use protocols such as Session Initiation Protocol, Skinny Client Control Protocol or one of various proprietary protocols such as that used by Skype. IP phones can be simple software-based Softphones or purpose-built hardware devices that appear much like an ordinary telephone or cordless phone or an ATA (analog telephony adapter) which allows to reuse ordinary PSTN phones. One of the primary motivations for implementing such a system is the lower calling cost. When calling other IP phones over the internet one only pays for the usually fixed cost internet bandwidth.