Abstract: A method and apparatus for generating a bill image in a computer implemented billing system for a telecommunications network which includes providing a plurality of discounts for a customer retroactively applied to calls made during a billing period in which the discounts apply. The method includes loading qualification criteria for the plurality of discounts and customer account details into the billing system, generating a data structure defining a time line over at least part of which the discounts are operative, the time line being divided in dependence on the qualification criteria and customer account details into a number of segments each of which corresponds to a period during which a respective version of the discounts were operative, accumulating charges for the calls made during each segment, and calculating the appropriate discounts from the accumulated charges for each segment for calls which qualify for the discounts.