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This allows a number of phones to be operated using a relatively low number of public telephone lines. Internal calls can be placed free of charge without using the public telephone network. Depending on the type of telephone system and the configuration, internal lines can be called directly from outside by using a direct dial in number, or indirectly through an operator. ### The various features of private automatic branch exchanges Private automatic branch exchanges typically offer a number of features to make communicating significantly more convenient. Key components include toggle, conference calls and picking up calls. Toggle is used to switch back and forth between multiple calls. With call pick-up you can answer a call to an unstaffed phone from your own phone. With conference call several subscribers can be included in a call and everybody can talk with all parties to the call. Other typical features of private automatic branch exchanges are callback, call waiting, call transfer and call forwarding. ### The technology of private automatic branch exchanges As telephone and communication technology advanced, the technology and functions of private automatic branch exchanges also changed significantly. In the early days, private automatic branch exchanges worked analogue and could connect to the public telephone network via several parallel lines. With digitisation and the introduction of [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn), private automatic branch exchanges were often connected using so-called ISDN primary [multiplex connections](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/multiplex-connections), which allowed 30 parallel connections with the public telephone network. By switching telephony to [Voice over IP](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) communication, software-based functions of extensions can now be implemented through standard servers. These telephone systems signal via IP protocol with [SIP](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sip) to communicate with phones or the public telephone network. Voice over IP private automatic branch exchanges to be run completely cloud-based. In this case the actual business site doesn't require separate hardware for the private automatic branch exchange. An internet connection with sufficient bandwidth is all that's needed to use all the features of the cloud-based system. ### Related Articles ### Public Switched Telephone Network (PSTN) A Public Switched Telephone Network, or PSTN for short, refers to a telecommunications network which allows subscribers at different sites to communicate by voice. The term plain old telephone service (POTS) is also frequently used. The features of a PSTN are: * Subscribers can be connected by entering telephone numbers * The existing connections are primarily used to transmit speech information * After hanging up the connection is closed and the resources used become available to other subscribers The term public switched telephone network is primarily used for public [landlines](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/dedicated-line). With traditional analogue landlines speech is transmitted analogue, a [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) network transmits speech information digitally. [Public Switched Telephone Network (PSTN)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn) ### Session Initiation Protocol (SIP) The acronym SIP stands for Session Initiation Protocol and refers to a TCP/IP-based network protocol which can be used to establish and control communication connections of several subscribers. SIP is often used in [Voice-over-IP telephony](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) to establish the connection for telephone calls. The key features of Session Initiation Protocol are specified in RFC3261. Since Session Initiation Protocol is an open standard, the protocol is widely used worldwide. Almost all IP-based [telephone systems](https://web-production.nfon.com/gb/products/cloudya) have SIP. 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