Voice communication systems are known in which precisely one terminal (telephone) is normally connected to each subscriber line. If it is possible to access a subscriber line from a plurality of terminals, however, such a line is referred to as MULAP (Multiple Line Appearance). Such circuits are frequently used for workgroups, where any employee can take calls arriving on a particular line. In this case, each terminal usually has a plurality of such MULAP lines connected to it which can be reached by internal and external subscribers on the respective call numbers (the “MULAP numbers”). Thus, in a company's sales department, for example, separate MULAP lines can be set up for order inquiries, technical information, delivery information and invoicing. Every terminal on which these MULAP lines are available then has a signal lamp and an access key (MULAP key) for each line. As soon as a customer calls on one of the MULAP lines, this call is signaled audibly on all of these terminals, the lamp associated with the line flashes and any subscriber in the workgroup can take the call by pressing the appropriate key.
Conversely, before dialing a call number, the employee can use the MULAP key for the purpose of specifically selecting a particular line. This line is then attributed the call charges, for example, the MULAP number for this line is transmitted to the called party, and the line is signaled to other callers as being “busy” for the duration of the call.
To control and monitor subscriber lines in private communication installations, special application programs installed on PCs are increasingly being used today. Such applications, also referred to as CTI (Computer Telephony Integration) applications, report incoming calls, for example, on the PC screen and in so doing indicate—according to availability—the caller's call number and name, the latter being ascertained from a telephone directory on the basis of this call number. In addition, CTI applications can also be used to set up connections by virtue of the ability to “dial” from address books and databases by “mouse click”. For these purposes, the PC or a central server existing in the computer network is connected to the telephone terminal or to the communication installation so that dialing commands, state and call information can be interchanged. At the communication installation end, these connections normally use standardized interfaces, for example the CSTA (Computer Supported Telephony Application) interface. Such interfaces use a standardized command set which, depending on the version (in the case of CSTA the term “phase” is also used), supports various commands of different performance.
A drawback which has been found with the known communication installations and CTI applications is that MULAP lines can be controlled and monitored using CTI applications only with difficulty or incompletely. One reason for this is that CTI applications and the standardized interfaces are intended for controlling and monitoring individual terminals and not for MULAP lines or groups. To monitor MULAP lines, a CTI application therefore needs to monitor a plurality of call numbers simultaneously, inter alia a specially set-up “virtual” call number (i.e. an apparent subscriber line to which no terminal is connected) for ascertaining the call state and also an internally allocated call number for the master line for each MULAP line, which is not known to the user of the system. This call number is required for setting up connections from the CTI application. Customary CTI applications are not set up to control and monitor subscriber lines and terminals having a plurality of associated call numbers, however.