X-Lite is a free SIP based softphone by CounterPath works on Windows, macOS, iOS and Android. X-Lite offers feature rich software for your personal as well as commercial use. The commercial version of CounterPath’s softphone is known as Bria.
Combining voice and video calls in a user-friendly interface, CounterPath’s X-Lite helps you seamlessly transition from a traditional phone environment into the world of Voice over IP. You can download it for free from their official website here.
Unfortunately X-Lite is not available for Linux platforms as of now. For desktop you have two options macOS and Windows. For mobile platforms, both iOS and Android versions are available.
To install it on Windows, just download the installer .exe file from here and follow the simple instructions to install.