This International Standard will provide users of voice messaging systems with a consistent mode of interaction in a way that is independent of the underlying system implementations. The interface is based on a set of design guidelines annexed to this International Standard. The interface supports the ability of all users described in the user-system model (see Figure 2) to access the features of voice messaging systems. In a call answering application, the interface allows callers to leave messages from all types of telephones. It also allows callers to access additional features through the use of DTMF devices. In the voice mail application, through the use of DTMF devices, the interface allows subscribers to send and receive voice messages, and to manage stored data, and allows non-subscribers to leave voice messages for subscribers. This International Standard addresses the following six functional areas: • a caller leaving a voice message, when the call is answered by a voice messaging system call answering facility; • a caller leaving a voice message, when a message is sent to a subscriber by a subscriber or nonsubscriber through direct messaging; • a subscriber listening to and processing voice messages received; • a subscriber creating and sending voice messages through the voice mail application; • the sending and receiving of messages via voice message delivery applications; and • the use of voice bulletin boards. Within these functional areas, only certain features are defined in this International Standard. However, standard-conforming systems are not limited to these functions and features, and this International Standard does not preclude alternative methods of invoking features specified in this International Standard, providing that these alternatives do not conflict with the standard interface specified for other features covered in this International Standard. In this International Standard, the direct messaging functional area is covered in the clause specifying the call answering application. In addition, this International Standard specifies two requirements to be satisfied in all voice messaging contexts, not just the application contexts specified earlier in this clause: the use of # as a delimiter (see 5.6.1), and the access to and presence of the control menu (see 5.6.2) and its associated functionality. This International Standard does not address the user/system interface for administrators, who have responsibility for the management and maintenance of the voice messaging system. This International Standard also does not address the proactive method, if any, employed by a voice messaging system to notify a user that a voice mailbox contains a message. Notification is, at present, typically achieved by a message waiting light, a distinctive dial-tone, or a pager device. This International Standard does not specify a non-DTMF user interface for Integrated Services Digital Network (ISDN) terminal access to voice messaging applications; however, if a user's ISDN terminal or switching equipment has the capability for full simulation of DTMF tones after call connection, the user interface specified in this International Standard will operate a conforming voice messaging system. Figure 1 shows a taxonomy of telephone-based services, with shaded boxes indicating the services within the scope of this International Standard: