Call Detail Record files or CDRs are files that contain the details of all the calls made.

Those downloaded from BT are simple comma-separated files, with the field or column names in the first record.

The BT files have the following columns :-

• Charge Code - The type of call; 'Local', 'Mobile', 'Ring Back' etc.

• Installation No - The number of phone making the call.

• Line No - The line number.

• Charge Card No - The charge card number.

• Date - The date on which the call was made.

• Time - The time when it was made.

• Destination - The name of the place, exchange or service called.

• Called No - The number called.

• Duration - The duration of the call.

• Txt Direct Rebate - Any ideas?

• Cost - The cost of the call in £.p to a tenth of a penny.

• Code - A user-defined code such as 'Personal', 'Business' etc.

• Called Description - A user-defined description of the number called.