A method for obtaining an identifier of a visited public land mobile network, V-PLMN, in order to register a user equipment, UE, which has roamed to the V-PLMN, to an IP multimedia subsystem, IMS, network, the method performed by a proxy-call session control function, P-CSCF, and comprising: receiving (S801, S901), by the P-CSCF, a request message to register to the IMS network from the UE; based on that the request message is received from the UE, (i) holding (S803-S805), by the P-CSCF, the request message and transmitting, by the P-CSCF, a temporary register message to an interrogating-CSCF, I-CSCF, or (ii) transmitting (S901), by the P-CSCF, the request message to the I-CSCF; receiving (S807, S903), by the P-CSCF, a response message from the I-CSCF in response to transmission of the request message or the temporary register message; (i) transmitting (S907), by the P-CSCF, the response message to the UE based on that the response message is a message for allowing a registration to the IMS network of the UE, or (ii) discarding (S811), by the P-CSCF, the response message based on that the response message is a 401 unauthorized message; generating (S811, S909), by the P-CSCF, an interface for transmitting and receiving data with respect to a Policy and Charging Rule Function, PCRF, of home PLMN, H-PLMN; and receiving (S817, S915), by the P-CSCF, the identifier of the V-PLMN from the PCRF through the interface, wherein the request message and the response message are received from the UE or transmitted to the UE via a Serving Gateway, S-GW, of V-PLMN and a Packet Data Network Gateway, P-GW, of H-PLMN. receiving (S801, S901), by the P-CSCF, a request message to register to the IMS network from the UE; based on that the request message is received from the UE, (i) holding (S803-S805), by the P-CSCF, the request message and transmitting, by the P-CSCF, a temporary register message to an interrogating-CSCF, I-CSCF, or (ii) transmitting (S901), by the P-CSCF, the request message to the I-CSCF; receiving (S807, S903), by the P-CSCF, a response message from the I-CSCF in response to transmission of the request message or the temporary register message; (i) transmitting (S907), by the P-CSCF, the response message to the UE based on that the response message is a message for allowing a registration to the IMS network of the UE, or (ii) discarding (S811), by the P-CSCF, the response message based on that the response message is a 401 unauthorized message; (i) transmitting (S907), by the P-CSCF, the response message to the UE based on that the response message is a message for allowing a registration to the IMS network of the UE, or (ii) discarding (S811), by the P-CSCF, the response message based on that the response message is a 401 unauthorized message; generating (S811, S909), by the P-CSCF, an interface for transmitting and receiving data with respect to a Policy and Charging Rule Function, PCRF, of home PLMN, H-PLMN; and receiving (S817, S915), by the P-CSCF, the identifier of the V-PLMN from the PCRF through the interface, wherein the request message and the response message are received from the UE or transmitted to the UE via a Serving Gateway, S-GW, of V-PLMN and a Packet Data Network Gateway, P-GW, of H-PLMN. <EOS>