Method for selecting P-CSCF and transmitting SIP message in wireless communication system and device for same

1. A method for transmitting a session initiation protocol (SIP) message by a user equipment (UE) in a wireless communication system, the method comprising:

establishing, by the UE which is out-coverage, a connection with a UE-to-network relay;

receiving an E-UTRAN (evolved-UMTS terrestrial radio access network) Cell Global Identifier (ECGI) from the UE-to-network relay;

comparing a first PLMN (public land mobile network) ID prior to connection with the UE-to-network relay with a second PLMN ID identified by the ECGI;

selecting an address of a proxy call session control function (P-CSCF) by considering a PLMN change, wherein the address is a first address of a first P-CSCF when the first PLMN ID is different from the second PLMN ID, and the address is a second address of a second P-CSCF when the first PLMN ID is same with the second PLMN ID; and

transmitting, by the UE, the SIP message to the P-CSCF having the selected address through the UE-to-network relay,

wherein the first address is received through a DHCP (Dynamic Host Configuration Protocol) message from the UE-to-network relay which performs a function of the DHCP server, and the second address is an address used in-coverage.

2. The method according to claim 1 , wherein the DHCP message is a DHCP offer or DHCP ack.

3. The method according to claim 1 , wherein the P-CSCF address is P-CSCF of a home network of the UE-to-network relay if the UE-to-network relay performs a packet data network (PDN) connection in a HR (Home-Routed) mode, and is P-CSCF of a visited network of the UE-to-network relay if the UE-to-network relay forms a PDN connection in a LBO (Local Breakout) mode.

4. The method according to claim 1 , wherein the UE uses the P-CSCF address stored therein if PLMN prior to connection with the UE-to-network relay is the same as PLMN of the UE-to-network relay.

5. The method according to claim 1 , wherein the P-CSCF address is one of IPv4 address, IPv6 address and FQDN (Fully Qualified Domain Name).

6. The method according to claim 1 , wherein the SIP message is a SIP register message.

7. A user equipment (UE) which is out-coverage for transmitting a session initiation protocol (SIP) message in a wireless communication system, the UE comprising:

a transceiver; and

a processor operatively coupled with the transceiver,

wherein the processor establishes a connection with a UE-to-network relay, receives E-UTRAN (evolved-UMTS terrestrial radio access network) Cell Global Identifier (ECGI) from the UE-to-network relay, compares a first PLMN (public land mobile network) ID prior to connection with the UE-to-network relay with a second PLMN ID identified by the ECGI, selects an address of a proxy call session control function (P-CSCF) by considering a PLMN change, wherein the address is a first address of a first P-CSCF when the first PLMN ID is different from the second PLMN ID, and the address is a second address of a second P-CSCF when the first PLMN ID is same with the second PLMN ID, and transmits the SIP message to the P-CSCF having the selected address through the UE-to-network relay, and the P-CSCF corresponds to a P-CSCF address acquired through the UE-to-network relay or a P-CSCF address stored in the UE,

wherein the first address is received through a DHCP (Dynamic Host Configuration Protocol) message from the UE-to-network relay which performs a function of the DHCP server, and the second address is an address used in-coverage.