A softphone is a phone app on a laptop or a mobile that carries a person's office number, so that they can make and take office calls without a desk phone. "Soft" refers to software: the phone is a program, not a device. A softphone is a part of a cloud phone system, and many office PBX systems offer one too.
How does it work? When the employee opens the app, it signs in to the phone system with her username and password. The app then tells the system that her extension can now be reached at the laptop's current internet address. This is called registering. From then on, when someone calls her extension, the system sends the call to wherever the app is registered: at home, in the office or in a hotel.
Say a sales executive at a tile manufacturer in Morbi is working from home on a Saturday. She opens the softphone on her laptop at 9:30 a.m., and it registers as extension 214. At 10:05, a tile dealer in Indore calls the company's main number. The phone system rings the four people in the sales group, and her laptop shows the dealer's number. She answers with her headset. The dealer asks the price of 600 boxes of floor tiles. She needs the latest stock figure, so she transfers the call to the warehouse, extension 205, and hangs up. On Monday, she takes calls in the office on the same app, with the same number.
Her voice travels as data, using VoIP. A codec, a method of compressing sound, turns her speech into small packets of data. A common codec for clear calls needs well under 100 kilobits a second, far less than a video. So an ordinary home broadband or mobile data connection is enough. If the connection is poor, packets arrive late or are lost, and her voice breaks up. Most softphones show a warning when this happens.
A softphone usually does more than a desk phone. It shows which colleagues are busy, holds the company directory, and opens team chat and video meetings from the same window. After each call, a transcript and summary of the call can appear in the same app.
A softphone is different from the ordinary calling app on a mobile. The ordinary app uses the mobile's own number and the operator's network. A softphone uses the office number, and it reaches the company's phone system over the internet.