In order to install the SightCall plugin on macOS, you will have to allow the package installer to run.
The first step is to open the "System Preferences" and go to "Security and Privacy":
Click the lock icon in the bottom left to make changes:
Then go back to the Downloads folder (or wherever the installer file was downloaded) and open it:
Press on "OK" and then go back to the System Preferences/Security & Privacy window, and click on "Open Anyway":
Then click on "Open" (in white) in the installer window:
You can now follow through the installer steps:
Once installed, you might need to reload the page to start a call.
On your first call, you will have to allow the "SightCall" app to access your microphone and camera