A method for ascertaining active communication sessions may include receiving a request to ascertain active communication sessions by a communication session information server; transmitting respective challenges to determine whether communication sessions are currently active from the communication session information server to at least two communication session servers; receiving respective challenge responses from the at least two communication session servers, which indicate one or more active communication sessions controlled by the respective communication session server or indicate that such communication sessions currently do not exist, by the communication session information server; and transmitting a first response, which indicates one or more active communication sessions controlled by the at least two communication session servers or indicates that such communication sessions currently do not exist, by the communication session information server.