Special facilities of the extension. charge-limit: This value determines if the maximum amounts available for calling is enabled. call-waiting: Call waiting signal for analog terminals. ISDN terminal devices use their own special procedures for this. announcement: Enables this phone to receive announcement calls. intercom: Allows intercom calls to the system telephone or to another identical ISDN telephone. silent-phone-intern: Enables a silent phone for incoming internal calls. silent-phone-extern: Enables a silent phone for incoming external calls. direct-call : Enables a direct call. parallel-ringing: Enables parallel ringing.