Method for participating in an Internet Protocol Multimedia Subsystem, hereinafter abbreviated as "IMS", registration and session initiation for a user equipment, hereinafter abbreviated as "UE", the UE having a subscription identifier comprising at least one of a IMSI and a MSISDN, the method including, in the following order: establishing, by a packet data network gateway, hereinafter abbreviated as "PDN-GW", an Internet Protocol connectivity access network, hereinafter abbreviated as "IP-CAN", session with a policy and charging rules function, hereinafter abbreviated as "PCRF", for the UE, by transmitting, to the PCRF, at least the subscription identifier of the UE, and an Internet Protocol, hereinafter abbreviated as "IP", address assigned to the UE; binding, by the PCRF, the subscription identifier to the IP address assigned to the UE for the IP-CAN session; upon the UE initiating the IMS registration, transferring, from a proxy call session control function, hereinafter abbreviated as "P-CSCF", to a serving call session control function, hereinafter abbreviated as "S-CSCF", an IP multimedia public identity, hereinafter abbreviated as "IMPU"; obtaining, by the S-CSCF, based on the IMPU, the subscription identifier of the UE from a user database; transferring, by the S-CSCF to the P-CSCF, the obtained subscription identifier of the UE; binding, by the P-CSCF, the subscription identifier of the UE to the IMPU; upon the UE initiating an IMS session associated with the IMPU, establishing, by the P-CSCF, an application function session towards the PCRF with the subscription identifier bound to the IMPU; and identifying, by the PCRF, the correct IP-CAN session by matching the subscription identifier received from the P-CSCF with the subscription identifier of the IP-CAN session. establishing, by a packet data network gateway, hereinafter abbreviated as "PDN-GW", an Internet Protocol connectivity access network, hereinafter abbreviated as "IP-CAN", session with a policy and charging rules function, hereinafter abbreviated as "PCRF", for the UE, by transmitting, to the PCRF, at least the subscription identifier of the UE, and an Internet Protocol, hereinafter abbreviated as "IP", address assigned to the UE; the subscription identifier of the UE, and an Internet Protocol, hereinafter abbreviated as "IP", address assigned to the UE; binding, by the PCRF, the subscription identifier to the IP address assigned to the UE for the IP-CAN session; upon the UE initiating the IMS registration, transferring, from a proxy call session control function, hereinafter abbreviated as "P-CSCF", to a serving call session control function, hereinafter abbreviated as "S-CSCF", an IP multimedia public identity, hereinafter abbreviated as "IMPU"; obtaining, by the S-CSCF, based on the IMPU, the subscription identifier of the UE from a user database; transferring, by the S-CSCF to the P-CSCF, the obtained subscription identifier of the UE; binding, by the P-CSCF, the subscription identifier of the UE to the IMPU; upon the UE initiating an IMS session associated with the IMPU, establishing, by the P-CSCF, an application function session towards the PCRF with the subscription identifier bound to the IMPU; and identifying, by the PCRF, the correct IP-CAN session by matching the subscription identifier received from the P-CSCF with the subscription identifier of the IP-CAN session.