Here is the honest warning. Since Zoiper Classic 2.43 is no longer officially supported or linked prominently on the main Zoiper website (which now pushes version 5+), you cannot just click the top Google result. Many third-party download sites bundle adware.
Choose Your Protocol: Most providers use SIP, but Zoiper Classic is well-known for its excellent IAX2 support. i zoiper classic 243 download better