Greeting the caller, reporting that all the operators are busy at the moment, plays music during waiting in queue, requests to stay on the line as the next available operator will pick up exactly your call.
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.