A method employs call detail records for developing a report on the usage of VoIP calling along a network path or through a network location. By utilizing, for example, a call-beginning record and a call-ending record for each connection, a report is created of the number of concurrent VoIP calls that a specified equipment or network path carries at any given instance by retrieving batches of stored call detail records, culling the records that are relevant to the specified equipment or network path, considering the culled records in a time sorted manner, and incrementing or decrementing a counter based on whether a considered record is a call-beginning record or a call-ending record, respectively. The state of the counter is reported, illustratively, as a line entry each time the counter's state is modified.