Methods, systems, and apparatus, including computer program products, in which a first telephone number is associated with a second telephone number, and a web-based advertisement is served for an advertiser that includes the first telephone number. A call placed to the first telephone number is received, and the call is routed to the second telephone number based on the association of the second telephone number with the first telephone number. A determination as to whether a call-through event in response to receiving the call placed to the first telephone number has occurred, and an advertiser associated with the second telephone number is charged a cost-per-call amount if the call-through event is determined to have occurred.