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One or several parallel lines can be used to connect to the telephone network. ### Private Branch Exchange functions 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](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/call-centre) often use the queue function of a telephone system. This will hold a call in a queue until an [agent](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/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. ### Technological advances in Private Branch Exchange Telephone systems have evolved from mechanical devices in the early telephone age to highly complex, digital devices. Due to [Voice-over-IP (VoIP)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) telephony the Private Branch Exchange is evolving more and more into a software-based solution which can be hosted on a local server or at a computer center in the [cloud](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/cloud). [Cloud-based telephone systems](/gb/en/products/cloudya) offer maximum flexibility, can be used regardless of location and do not require the company to provide hardware to operate the system. ### Related Articles ### Voice over Internet Protocol (VoIP) **What is VoIP?** The **Voice over Internet Protocol,** also called IP telephony, is a method of communication for making calls over a broadband Internet connection as an alternative to regular calls made over an analogue phone using the [PSTN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). These VoIP calls can happen entirely over the Internet but where required, gateways also link to the PSTN. This means that VoIP users can still connect to anyone with a telephone number including local, mobile, and international numbers. VoIP services can be accessed from a computer with a so-called [softphone,](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/softphones) from an IP phone, from a smartphone or from a conventional landline phone that is connected to a VoIP adapter. 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