Zoiper 3 was designed as a lightweight, cross-platform softphone that bridged the gap between simple dialers and full unified communication suites.
Zoiper 3.15 uses the old GPL + Commercial license. Read the EULA – notably, the free version requires a "disclaimer dialog" on every startup.
Zoiper is a softphone application (VoIP client) that allows users to make voice and video calls, send messages, and manage contacts using SIP and IAX protocols. It is available for Windows, macOS, Linux, iOS, and Android.