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[Write to us](/gb/request-information "Write to us") ### Multiple Subscriber Numbers (MSN) [What is ISDN?](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/msn#section-3) [What are MSNs used for?](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/msn#section-4) [How does placing calls an MSN compare with VoIP?](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/msn#section-5) # Multiple Subscriber Numbers (MSN) **What are MSNs?** Multiple Subscriber Numbers (MSNs) relate to the [Integrated Digital Services Network (ISDN).](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) They allow users to allocate different [phone numbers](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/national-international-phone-numbers) to different devices within the network. Like a landline, a complete MSN consists of a country and area code followed by the MSN number. ### msn's ![](https://a.storyblok.com/f/290788/ab2882b710/msns.jpg) ### What is ISDN? ISDN is the technology that brought digital functionality to the traditional phone network, otherwise known as the [Public-Switched Telephone Network (PSTN)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). In particular, ISDN enables the simultaneous transmission of different types of data, such as voice and video, over the PSTN. In the near future, ISDN will be decommissioned across Europe in favour of [voice-over-IP (VoIP)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) technology, which routes phone calls across the internet rather than the PSTN. In the UK, BT has already announced that ISDN will be decommissioned in the next few years, while the date set for decommissioning in Germany is 2018. VoIP technology is far more efficient and cost-effective than ISDN, which is one of the main reasons it is being decommissioned. ### What are MSNs used for? MSNs are used to ensure that every device on the network has a different phone number. Additionally, they allow users to make two voice calls simultaneously over a single ISDN connection. When a phone call enters the ISDN network, it is delivered to the specific MSN via the [S0 bus](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/s0-bus). NOTE: One device can have multiple MSN numbers allocated to it. ### How does placing calls an MSN compare with VoIP? VoIP solutions such as the [NFON Cloud Telephone System](/gb/en/products/cloudya) are much more flexible than MSNs. This is because MSNs are static numbers that are allocated to fixed points in the network, whereas VoIP connections allow users to log in from anywhere, using any device. This means that with MSNs, employees visiting different offices are not able to use their own phone numbers and can only communicate using local numbers at that office. With VoIP, they can simply log into the remote office network and use their own phone number to contact colleagues or customers. For more information about MSNs and other telephone technologies, or to discover the benefits of the NFON Cloud Telephone System, please review the articles below: ### Related Articles ### Integrated Services Digital Network (ISDN) In telecommunications technology, the abbreviation ISDN stands for the technical term "Integrated Services Digital Network" and refers to a digital standard for telephone networks. As the name implies, various types of communication services can be transmitted through this kind of network. In addition to voice calls, ISDN allows packet-switched or circuit-switched data transmission and fax transmission. In some countries, ISDN technology has taken over from analogue telephone networks, integrating all types of services into a common network for the first time. This digitisation has also enabled digital ISDN phone lines to be made available to end users. In comparison to an analogue connection, an ISDN subscriber line offers a whole range of advantages. The basic ISDN line allows multiple phones or communication [devices](https://web-production.nfon.com/gb/products/devices), with different numbers ([MSNs](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/msn)), to operate and use two parallel connections simultaneously. In addition, the ISDN connection also offers higher data transfer rates to an analogue connection using a modem, for example, when accessing the Internet. To utilise the ISDN line, end users need special ISDN-capable phones or faxes. The use of so-called [A/B converters](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/a-b-interface), linking analogue devices to a digital connection, is common. Meanwhile, other techniques such as DSL or cable connections are used for connecting to the internet. Nowadays, phone technology is evolving away from ISDN and towards IP telephony via [Voice over IP (VoIP).](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) [Integrated Services Digital Network (ISDN)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) ### S0 bus **What is an S0 interface?** An S0 interface (sometimes also called an S interface or S0 bus) allows traditional telephone users to connect to the [Integrated Services Digital Network (ISDN)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) and place calls over the [Public-Switched Telephone Network (PSTN)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). It is is similar to a [router](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/router) or modem that allows users to connect to the Internet, except that it provides connectivity into the traditional telephone network instead.  Note: Even though ISDN is digital, it does not make use of the internet. [S0 bus](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/s0-bus) ### 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. [Voice over Internet Protocol (VoIP)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) ### Telephone Connection A telephone connection is a standard part of telecommunications. In the following, we will first take a closer look at the term. 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