Enabling single radio voice call continuity (SRVCC) in a second access when a user equipment (UE) is using a first access

1. A method implemented by a network node, the method comprising:

receiving, via a user equipment (UE), a session initial initiation protocol (SIP) registration request; and

in response to receiving the SIP registration request:

obtaining non-access stratum (NAS) registration state information associated with the UE from a first core network node comprising a home subscriber server (HSS), the NAS registration state information indicating at least one of fourth-generation (4G) single radio voice call continuity (SRVCC) capability of the UE or fifth-generation (5G) SRVCC capability of the UE that is applicable when using a second access network supported by a second core network, wherein the UE is registered with a first IP Multimedia Subsystem (IMS) network that includes the network node using the SIP via the second access network supported by the second core network and wherein an indication of the at least one of 4G or 5G SRVCC capability of the UE is included in a NAS registration request originating from the UE; and

providing, to a second IMS network node, an indication of a single UE SRVCC capability for the UE.

2. The method of claim 1 , wherein the second core network comprises a 5G core network.

3. The method of claim 1 , wherein the network node comprises an MIS service centralization and continuity application server (SCC AS).

4. The method of claim 1 , wherein the second IMS network node comprises an access transfer control function (ATCF) node.

5. The method of claim 1 , wherein the second IMS network node is in a second IMS network.

6. The method of claim 1 , wherein providing the indication of a single UE SRVCC capability for the UE comprises sending, to the second IMS network node, a SIP message request containing SRVCC information.

7. The method of claim 1 , wherein the 4G UE SRVCC capability comprises a UE packet-switched (PS) to circuit-switched (CS) SRVCC capability.

8. A computing device implementing a network node, the computing device comprising:

a processor; and

a memory coupled to the processor, the memory storing instructions that, when executed, configure the processor to:

receive, via a user equipment (UE), a session initiation protocol (SIP) registration request; and

in response to receiving the SIP registration request:

obtain non-access stratum (NAS) registration state information associated with the UE from a first core network node comprising a home subscriber server (HSS), the NAS registration state information indicating at least one of fourth-generation (4G) single radio voice call continuity (SRVCC) capability of the UE or fifth-generation (5G) SRVCC capability of the UE that is applicable when using a second access network supported by a second core network, wherein the UE is registered with a first IP Multimedia Subsystem (IMS) network that includes the network node using the SIP via the second access network supported by the second core network and wherein an indication of the at least one of 4G or 5G SRVCC capability of the UE is included in a NAS registration request originating from the UE; and

provide, to a second IMS network node, an indication of a single UE SRVCC capability for the UE.

9. The computing device of claim 8 , wherein the second core network comprises a 5G core network.

10. The computing device of claim 8 , wherein the network node comprises an IMS service centralization and continuity application server (SCC AS).

11. The computing device of claim 8 , wherein the second IMS network node comprises an access transfer control function (ATCF) node.

12. A server-implemented method, comprising:

receiving a request from an IP Multimedia Subsystem (IMS) network node of an IMS network; and

in response to receiving the request, providing NAS registration state information associated with a user equipment (UE) to the IMS network node, the NAS registration state information indicating at least one of fourth-generation (4G) single radio voice call continuity (SRVCC) capability of the UE or fifth-generation (5G) SRVCC capability of the UE that is applicable when using a second access network supported by a second core network, wherein an indication of the at least one of 4G or 5G SRVCC capability of the UE is included in a NAS registration request originating from the UE, wherein the server is configured to store separate instances of UE SRVCC capabilities, the separate instances including indications of at least one of 4G UE SRVCC capability or 5G UE SRVCC capability that is applicable when using the second access network supported by the second core network, and wherein the UE is registered with the IMS network using a session initiation protocol (SIP) via an access network.

13. The method of claim 12 , wherein the NAS registration state information indicates both 4G UE SRVCC capability and 5G UE SRVCC capability.

14. The method of claim 12 , wherein the access network comprises one of: a network supported by a 3 rd Generation Partnership Project (3GPP) core network, a Next Generation Radio Access Network (NG-RAN) supported by a 5G core network, or an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access supported by an Evolved Packet Core (EPC) network.

15. The method of claim 12 , wherein the IMS network node comprises an IMS service centralization and continuity application server (SCC AS), and wherein the NAS registration state information is provided via a Sh interface associated with the IMS network node.

16. The method of claim 12 , wherein the 4G UE SRVCC capability comprises a UE packet-switched (PS) to circuit-switched (CS) SRVCC capability.

17. The method of claim 12 , wherein the server comprises at least one of a home subscriber server (HSS) and unified data management (UDM).

18. The method of claim 17 , wherein the server comprises the HSS and a collocated UDM.