The invention relates to a method of listening in to calls between two or more subscribers of a communications network comprising one or more IP networks as well as to a listening server, a call control server, and an IP router for carrying out the method. When the listening server detects a request to monitor one of the subscribers, it determines at least one call control server assigned to the subscriber. Further, at least one IP router is determined which is responsible for a network access point of the subscriber to the IP network or IP networks. The listening server causes the at least one determined call control server to acquire call data associated with calls in which the subscriber is involved. Further, the at least one determined IP router is caused to acquire contents data associated with calls in which the subscriber is involved.