UE network mobility during IMS

1. A method of wireless communication for a user equipment (UE) having a connection to a first network providing an Internet Protocol (IP) address for the UE, comprising:

receiving, via the first network, a message indicating an incoming IP Multimedia Subsystem (IMS) call;

determining, by the UE, that a second network is preferred over the first network for the incoming IMS call based on a priority for a type of IMS call indicated by the message;

transmitting one or more IMS call setup messages on the first network at an IP layer;

changing, by the UE, the connection from the first network to the second network that uses a same identifier for the UE; and

completing setup of the IMS call on the second network using the same identifier for the UE.

2. The method of claim 1 , wherein the UE uses a single radio for the first network and the second network.

3. The method of claim 1 , further comprising registering with the first network and the second network using a same credential, wherein the first network and the second network provide the identifier based on the same credential.

4. The method of claim 1 , wherein the identifier is an IP address.

5. The method of claim 1 , wherein changing the connection comprises:

dropping, by the UE, a radio resource control (RRC) connection to the first network; and

initiating, by the UE, a new RRC connection to the second network.

6. The method of claim 1 wherein changing the connection comprises:

transmitting, by the UE, a measurement report indicating conditions for a change to the second network are satisfied in response to the transmitting the one or more IMS call setup messages on the first network;

receiving a handover command from the first network to handover to the second network in response to the measurement report; and

switching to the second network in response to the handover command.

7. The method of claim 1 , wherein the changing the connection is in response to sending a 100 TRYING message, and wherein the changing of the connection occurs before sending a 183 Session Progress message.

8. The method of claim 1 , wherein the changing the connection is in response to sending a 200 PRACK OK message, and wherein the changing of the connection occurs before receiving an UPDATE message.

9. The method of claim 1 , wherein the changing the connection occurs after completing setup the IMS call on the first network at a fixed time during the IMS call or when silence is detected during the IMS call.

10. The method of claim 1 , further comprising establishing a first non-access stratum (NAS) context for the UE on the first network and a second NAS context for the UE on the second network before receiving the message indicating the incoming IMS call, wherein changing the connection to the second network uses the second NAS context.

11. The method of claim 1 , wherein the first network is a shared network and the second network is an operator's public land mobile network.

12. The method of claim 1 , wherein the first network is a shared network and the second network is a wireless local area network (WLAN).

13. The method of claim 1 , wherein the first network is a wireless local area network (WLAN) and the second network is an operator's public land mobile network.

14. The method of claim 1 , wherein the type of IMS call for the incoming IMS call is one of a voice call, a video telephony call, an emergency call, a real time text call, or a rich communication services session.

15. An user equipment (UE) for wireless communication, comprising:

a transceiver;

a memory; and

at least one processor coupled to the transceiver and the memory, the memory comprising instructions executable by the at least one processor to cause the UE to:

receive, via a first network, a message indicating an incoming IP Multimedia Subsystem (IMS) call, wherein the UE has a connection to the first network providing an Internet Protocol (IP) address for the UE;

determine, by the UE, that a second network is preferred over the first network for the incoming IMS call based on a priority for a type of IMS call indicated by the message;

transmit one or more IMS call setup messages on the first network at an IP layer;

change the connection from the first network to the second network that uses a same identifier for the UE; and

complete setup of the IMS call on the second network using the same identifier for the UE.

16. The UE of claim 15 , wherein the transceiver includes a single radio for the first network and the second network.

17. The UE of claim 15 , wherein the at least one processor is further configured to cause the UE to register with the first network and the second network using a same credential, wherein the first network and the second network provide the identifier based on the same credential.

18. The UE of claim 15 , wherein the identifier is an IP address.

19. The UE of claim 15 , wherein the at least one processor configured to cause the UE to change the connection is further configured to cause the UE to:

drop a radio resource control (RRC) connection to the first network; and

initiate a new RRC connection to the second network.

20. The UE of claim 15 , wherein the at least one processor configured to cause the UE to change the connection is further configured to cause the UE to:

transmit a measurement report indicating conditions for a change to the second network are satisfied in response to transmitting the one or more IMS call setup messages;

receive a handover command from the first network to handover to the second network in response to the measurement report; and

switch to the second network in response to the handover command.

21. The UE of claim 15 , wherein the change of the connection is in response to sending a 100 TRYING message, and wherein the change of the connection occurs before sending a 183 Session Progress message.

22. The UE of claim 15 , wherein the change of the connection is in response to sending a 200 PRACK OK message, and wherein the change of the connection occurs before receiving an UPDATE message.

23. The UE of claim 15 , wherein the change of the connection occurs after completing setup the IMS call on the first network at a fixed time during the IMS call or when silence is detected during the IMS call.

24. The UE of claim 15 , wherein the at least one processor is further configured to cause the UE to establish a first non-access stratum (NAS) context for the UE on the first network and a second NAS context for the UE on the second network before receiving the message indicating the incoming IMS call, wherein changing the connection to the second network uses the second NAS context.

25. A user equipment (UE) for wireless communication, comprising:

means for receiving, via a first network, a message indicating an incoming IP Multimedia Subsystem (IMS) call, wherein the UE has a connection to the first network providing an Internet Protocol (IP) address for the UE;

means for determining, by the UE, that a second network is preferred over the first network for the incoming IMS call based on a priority for a type of IMS call indicated by the message;

means for transmitting one or more IMS call setup messages on the first network at an IP layer;

means for changing, by the UE, the connection from the first network to the second network that uses a same identifier for the UE; and

means for completing setup of the IMS call on the second network using the same identifier for the UE.

26. A non-transitory computer-readable medium storing code for a user equipment (UE) having a connection to a first network providing an Internet Protocol (IP) address for the UE, the code executable by at least one processor to cause the UE to:

receive, via the first network, a message indicating an incoming IP Multimedia Subsystem (IMS) call;

determine, by the UE, that a second network is preferred over the first network for the incoming IMS call based on a priority for a type of IMS call indicated by the message;

transmit one or more IMS call setup messages on the first network at an IP layer;

change, by the UE, the connection from the first network to the second network that uses a same identifier for the UE; and

complete setup of the IMS call on the second network using the same identifier for the UE.