Abstract: Certain telephone calls cannot be billed properly due to the failure to detect a valid answer signal from the called customer. This disclosure presents an arrangement for detecting irregular calls that are recognized initially by a momentary off-hook signal from the called end of the trunk which is not quickly followed by a release of the trunk. When a momentary off-hook signal is received, the holding time of the trunk is measured. If the holding time exceeds a predetermined threshold a record is made of the lines connected to the trunk.The calls are also categorized by called destination to detect fraudulent calls which are frequently directed to selected destinations.