Method for detecting and correlating copies of Session Initiation Protocol, SIP, and Real-time Transport Protocol, RTP, flows, respectively, belonging to a communication session of an intercepted user, a target, said session being established via an Internet protocol Multimedia Subsystem, IMS, having a well-known Access Point Name, APN, which session is intercepted in an IMS domain by an Interception Access Point, IMS/IAP, and in an EPS domain by an Interception Access Point, EPS/IAP, by which interception result in the generation of copies of the session's SIP and RTP flows comprising flow identity information defined by a set of identifiers, triplet, comprising Lawful Interception Identity LI-ID and one of assigned IP user addresses IP_SIP or IP_RTP of the target, and identical content, said copies of SIP and RTP flows being received by a mediation function of a Lawful Interception, LI, system, wherein the method comprises the steps of: - Determining a unique IMS Communication Identity Number, IMS CIN, and a corresponding correlation set of identifiers comprising Correlation Number of default Bearer CN_def_B, Correlation Number of dedicated Bearer CN_ded_B, and IMS Correlation Number CN_IMS for each intercepted communication session at Packet Data Protocol, PDP, context/Bearer set up and establishment event of the communication session via the IMS well-known APN; - Storing each unique IMS CIN together with its correlation set for an intercepted communication session; - Correlating a SIP or RTP flow received from one domain to the same SIP or RTP flows of the same communication session received from the other domain by comparing the flow identity information of the received flow to the stored correlation sets for identifying a matching correlation set and its unique IMS CIN; - Sending to a Law Enforcement Agency, LEA, requesting for LI of the target said received SIP or RTP flow comprising said identified unique IMS CIN for a matching correlation set enabling the LEA to match SIP and RTP flows of the same communication session regardless intercepted in an IMS domain or an EPS domain. - Determining a unique IMS Communication Identity Number, IMS CIN, and a corresponding correlation set of identifiers comprising Correlation Number of default Bearer CN_def_B, Correlation Number of dedicated Bearer CN_ded_B, and IMS Correlation Number CN_IMS for each intercepted communication session at Packet Data Protocol, PDP, context/Bearer set up and establishment event of the communication session via the IMS well-known APN; - Storing each unique IMS CIN together with its correlation set for an intercepted communication session; - Correlating a SIP or RTP flow received from one domain to the same SIP or RTP flows of the same communication session received from the other domain by comparing the flow identity information of the received flow to the stored correlation sets for identifying a matching correlation set and its unique IMS CIN; - Sending to a Law Enforcement Agency, LEA, requesting for LI of the target said received SIP or RTP flow comprising said identified unique IMS CIN for a matching correlation set enabling the LEA to match SIP and RTP flows of the same communication session regardless intercepted in an IMS domain or an EPS domain.