Method and device of communicating with a LTE network and a NR network

1. A communication device for communicating with a long-term evolution (LTE) network and a new radio (NR) network, comprising:

at least one storage device; and

at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:

registering to the LTE network and the NR network;

performing a first IP multimedia subsystem (IMS) registration via the LTE network but not via the NR network, when registering to the LTE network and the NR network; and

performing a second IMS registration via the NR network, when failing to perform the first IMS registration via the LTE network;

suspending a NR operation when having an IMS service via the LTE network, after performing the first IMS registration; and

resuming the NR operation, after finishing the IMS service.

2. The communication device of claim 1 , wherein the instructions further comprise:

establishing a Packet Data Network (PDN) connection with the LTE network by performing a PDN connectivity procedure with the LTE network.

3. A communication device for communicating with a long-term evolution (LTE) network and a new radio (NR) network, comprising:

at least one storage device; and

at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:

registering to the LTE network and the NR network;

performing a first IP multimedia subsystem (IMS) registration via the LTE network and performing a second IMS registration via the NR network, when registering to the LTE network and the NR network;

suspending a NR operation when having an IMS service via the LTE network, after performing the first IMS registration and the second IMS registration; and

resuming the NR operation, after finishing the IMS service.

4. The communication device of claim 3 , wherein the instructions further comprise:

establishing a Packet Data Network (PDN) connection with the LTE network by performing a PDN connectivity procedure with the LTE network; and

establishing a Protocol Data Unit (PDU) Session with the NR network by performing a PDU Session establishment procedure with the NR network.

5. The communication device of claim 3 , wherein the instructions further comprise:

ignoring a paging for a mobile terminating call from the NR network when having an IMS voice call or an IMS video call via the LTE network, after performing the first IMS registration and the second IMS registration.

6. The communication device of claim 3 , wherein the instructions further comprise:

rejecting a mobile terminating call from the NR network when having an ongoing IMS voice call or an ongoing IMS video call via the LTE network, after performing the first IMS registration and the second IMS registration.

7. The communication device of claim 3 , wherein the instructions further comprise:

rejecting a mobile terminating call from the LTE network when having an ongoing IMS voice call or an ongoing IMS video call via the NR network, after performing the first IMS registration and the second IMS registration.