This is intended to be used with Cisco IP phone systems (typically Cisco Unified IP-phone 9951 and 8961).
Set in the settings the address of the web interface to the IP phone system (typically https://<IP of="" phone="" system="">/webdialer/Webdialer)
Set your caller ID (the same that you would need for the web interface of the phone system).
Log-in with Firefox into this phone system.
Then you can right click on any phone system to call a number.