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However, international calls and calls between or to cell phones are still billed based on the length of the call. ### The billing unit and its syntax A certain syntax has been established for the billing unit. The unit is represented by two numbers separated by a forward slash. The first number represents the length of the first unit in seconds, the second number represents the length of all further units. Hence, the unit "60/1" means the first minute (60 seconds) of a call is always charged in full, even if the call was less than a minute. After the first minute, all additional fees are billed by the second. A 60/60 unit charges the full price per minute for each minute and part thereof. The different service providers frequently use the billing units 60/60, 60/10 or 60/1. ### The billing unit and its metering pulse in analog or digital phone networks Analog or digital phone networks allow a metering pulse based on the billing unit to be transmitted to a [device](/gb/en/products/devices) over the network. For instance, in the age of analog phones a subscriber's meter or a phone with built-in subscriber's meter were quite common. Even phone booths used the metering pulse to indicate fees. The metering pulse itself consists of an power frequency above the speech frequency range of 300 to 3400 Hertz and could often be heard as a clicking in the line. If the fee device receives this type of pulse, it converts it into the respective fee information. On a digital [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) network or cell phone networks, fee information based on the billing unit can be provided using the so-called AOC information. An ISDN connection uses the D-channel, which is separate from the speech channel. ### 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. 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