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") ![](/images/getstarted-writetous-navbar-int_ZY7MOf.webp) ### Write to us Fill out our request form. Our experts will respond as soon as possible. [Write to us](/en/request-information "Write to us") ### Telephone number block [The structure of a telephone number with basic telephone number and telephone number block](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/telephone-number-block#section-3) [The telephone number block and Voice-over-IP telephony](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/telephone-number-block#section-4) To facilitate convenient direct dialing and easy management of direct dial numbers, large [telephone systems](t3://page?uid=16) commonly use a separate telephone number block. This is a defined set of successive telephone numbers attached to a basic telephone number. ### The structure of a telephone number with basic telephone number and telephone number block The entire telephone number used to phone a subscriber on a telephone system consists of the basic telephone number and a number in a telephone number block. If the telephone number block is single digit, the direct dial numbers can be 0 - 9. The entire telephone number can then be for example: 2378 -2 2378 -3 2378 -4 The direct dial number 0 of often used as a direct number to the switchboard. Single digit telephone number blocks allow up to ten successive direct dial numbers. Likewise, two-digit blocks can address up to 100 terminals, and three-digit blocks up to 1,000 terminals. The telephone numbers in an assigned telephone number block can be managed and used within the telephone system completely at your own responsibility. The telephone number block is typically ordered from the telephone service provider. Here it is important to order an adequately sized telephone number block straight away. This ensures consecutive direct dial numbers can be continued in future expansions. ### The telephone number block and Voice-over-IP telephony Just as in [ISDN](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) system connections, telephone number blocks can also be used in [Voice-over-IP telephony (VoIP telephony)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip). VoIP system connections are much more flexible than conventional telephone connections. After all, telephone number blocks can be assigned to an [SIP trunk](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/sip-trunk) with a simple configuration. The hardware or lines typically do not need to be upgraded for a larger number of telephone numbers. ["Fixed Mobile Convergence" (FMC)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/fmc) functions even allow assigning mobile terminals a fixed-line number from the telephone number block. When porting fixed-line numbers to Voice-over-IP providers it is important to note the prefix portion of the telephone number must remain assigned to the company’s registered office and is not arbitrary. ### Related Articles ### Integrated Services Digital Network (ISDN) [Integrated Services Digital Network (ISDN)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) ### Fixed Mobile Convergence (FMC) [Fixed Mobile Convergence (FMC)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/fmc) ### Voice over Internet Protocol (VoIP) PSTN stands for Public Switched Telephone Network and is generally known as the traditional phone network. It has been connecting people since the late 19th century, at first over copper telephone lines and later using fibre optic cables, satellites, and undersea cables. All of these are interconnected by switching centres which establish a dedicated connection – or circuit – between callers. Voice signals can then travel over this circuit. When the caller speaks into his handset, the caller’s phone translates the sound waves into electrical signals. These are transmitted over the PSTN (converted to optical signals for part of the way, so they can travel via fibre optic cables), and the phone at the other end of the line then translates the received signals back into sound. The arrival of **Voice over IP (VoIP)** technology – essentially, transmitting voice calls over the Internet rather than the PSTN – challenged traditional telephony and fundamentally changed how people communicate. 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