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.
Analog or digital phone networks allow a metering pulse based on the billing unit to be transmitted to a device 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 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.