A softphone is a software program for making telephone calls over the Internet using a general purpose computer rather than dedicated hardware.
The softphone can be installed on a piece of equipment such as a desktop, mobile device, or other computer and allows the user to place and receive calls without requiring an actual telephone set.
[1] Often, a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a handset with a display panel and buttons with which the user can interact.
Many service providers use the Session Initiation Protocol (SIP) standardized by the Internet Engineering Task Force (IETF).
A typical softphone has all standard telephony features (DND, Mute, DTMF, Flash, Hold, Transfer etc.)