All about Call Accounting

What is Call Accounting?

Call Accounting is the process of calculating the cost of telephone calls and allocating the costs to particular users.

Call Accounting is especially used by hotels, motels and other accommodation providers where guests are able to make telephone calls through a PABX or key telephone system and have the cost of the calls charged to their account.

It can also be used by any business or organisation that wants to keep track of its call costs and possibly attribute them to the department or cost centre from which the calls are made.

Professional businesses such as accountants and solicitors use call accounting to attribute calls to individual clients. In this case, it is not so much the cost of the call that matters but the duration and hence the cost of the professional's time. This application also requires the use of a client account code which must be keyed into the phone system at some stage during the call.

How does Call Accounting work?

When a call is made through a PABX or key telephone system, information about the call is collected and at the end of the call that information is transmitted to a computer or dedicated call accounting device. The information transmitted usually includes at least the originating extension, the number dialled, and the date, time and duration of the call. It may also include an account code and the cost (or a number of meter pulses that reflect the cost) of the call. In Australia, Telstra is the only carrier that can provide meter pulses.

Methods of data transmission

Various kinds of connection are used to transmit data between the PABX and the call accounting device:

RS232 serial interface - originally used to print call records directly to a serial printer, but now normally used to connect to a computer running appropriate call accounting software.

TCP/IP network interface - the usual mode of operation is for the PABX to create a TCP server socket and wait for an external device to connect. Alternatively the PABX may attempt to connect as a client to some specified external device acting as a server.

File interface - the PABX generates a file containing the call records which it places in a directory accessible to the call accounting software.

The stream of data generated by the PBX is variously referred to by different manufacturers as SMDR, CDR, or CIL.

Calculating the cost of a call

When meter pulses are being used, one pulse is generated at the beginning of each call. Then, except for local calls, additional pulses are generated at regular intervals depending on various factors such as the type of call (STD, mobile, IDD etc) and possibly the day of the week and/or the time of day. The call accounting software then calculates the cost of the call by multiplying the number of pulses by some suitable factor, such as 25 cents per pulse, or possibly on a stepped scale with a reduced factor for subsequent pulses.

In the absence of meter pulses, the call accounting software must first determine the type and destination of the call (STD, mobile, international etc) by using a table of number prefixes. It then looks up the type of call in another table to determine the flagfall charge and rate per minute and uses that information with the duration of the call to calculate the final cost.

Glossary of terms

PABX - Private Automatic Branch eXchange (also called PBX in some contries) - a telephone exchange that serves an individual business, as opposed to the public exchanges that serve the general public. The PABX connects via internal lines to extensions within the organisation and via external lines (or trunks) to the public network.

Key telephone system - a multiline telephone system in which the user manually selects an external line by pressing a key and then dials the required external number. The distinction between a PABX and a key system has become very fuzzy with many hybrid systems now being available.

CDR - Call Data Recording

CIL - Call Information Logging

SMDR - Station Message Detail Recording

PMS - Property Management System

PSTN - Public Switched Telephone Network - also called POTS - this is the familiar voice quality analogue telephone network

POTS - Plain Old Telephone Service

ISDN - Integrated Services Digital Network

Call accounting - calculation and attribution of call costs

Call billing - synonym for Call accounting

Call logging - strictly the recording of call data, but may also include Call accounting