A Private Branch Exchange interconnects devices and allows them to make internal calls without using the public switched telephone network. These internal calls are free of charge. When calling other subscribers in other locations via the telephone network the telephone system assigns one of the existing connections with the public switched telephone network to the respective device. This has the advantage of not requiring a separate connection to the public switched telephone network for each phone. The existing lines can therefore be used quite efficiently. The maximum number of parallel calls which can be made depends on the number of exchange lines.
From outside, the extensions on the telephone system can either be called via an extension suffix to the main number or a central switchboard. Call centers often use the queue function of a telephone system. This will hold a call in a queue until an agent connected to the telephone system is available and then transfers the call.
Many telephone systems are able to record a large volume of data and statistics to precisely analyze a company’s call volume. Some telephone systems allow the system administrator to assign different rights to different extensions, for instance for outward dialling. So expensive international calls, for example, can only be placed from specific extensions.