This is a device allowing a user to make VoIP calls using session initiation protocol (SIP). SIP is a telecommunication technology developed as a fast, easy standard for delivering IP-based data, voice, and video communications.
In the past, when you call an office building or business, you would either be transferred to the department you want to talk to, or given their extension number. This extension is usually a 2-3 digit code that when dialed, transfers you automatically to the particular person or department you want to speak with. The reason why this code works is because it is programmed into a PBX system.