Blink is a free (as in freedom) SIP client for Linux, Windows, and macOS. It is developed by AG Projects. It's mostly used on Linux, but it's also available for Windows and macOS. (Although we recommend MicroSIP for Windows)
We're going to assume that you have already created a SIP extension in the PBX and also downloaded and installed Blink.
Open Blink and click on the Accounts tab. Click on the + button to add a new account. The SIP URI will be something like sip:<extension>@domain.com. Let's go over all the other settings:
You can click on the re-register button to make sure that everything is working correctly.