It is a call which is made over internet protocol (VoIP). For example,
one can call using Google Voice to other recipient located miles away. Such
call can also be international.
The system of Classes of Service (COS) helps defining rights for one or another extension in the PBX or Centrex systems. For example, few groups require more space for storing voice messages, while other need the redirection of calls to cell phones, the third does not have to make calls outside of the office network. With the help of this system one can change and configure settings.