If you require a completely free SIP client without a commercial license key requirement, Linphone is an industry standard.
Given the risks and obsolescence, setting up the software is not recommended for practical use today. However, if you wish to explore its interface for purely academic or historical reasons on an isolated, offline virtual machine, the process would typically involve these steps from its heyday: eyebeam 15 202 license key
Older installers frequently crash on modern 64-bit systems because they attempt to write data to obsolete 32-bit registry paths. If you require a completely free SIP client
The EyeBeam 15202 License Key Guide: Safe VoIP Softphone Activation offline virtual machine