Method of handover of circuit-switched voice call to packet-switched voice call

1. A method comprising:

detecting or establishing a circuit-switched call over a cellular network;

establishing, over a cellular network, a voice-over-internet-protocol standby call in parallel to the circuit-switched call, wherein the voice-over-internet-protocol standby call is a standby, virtual call that includes an established end-to-end signaling path but utilizes no physical radio resources, wherein the end-to-end signaling path is ready for use after a cellular handover situation to an active voice-over-internet protocol call established using the end-to-end signaling path;

detecting the cellular handover situation;

when the detected cellular handover situation is to a first type of network:

activating the voice-over-internet-protocol standby call to become the active voice-over-internet protocol call, and

releasing resources of the circuit-switched call, when the active voice-over-internet protocol call is established; and

when the detected cellular handover situation is to a second type of network, wherein the second type of network is different from the first type of network:

performing the cellular handover using dual transfer mode signaling, and

maintaining the voice-over-internet protocol standby call in a standby state.

2. The method of claim 1 , wherein the method is coordinated between a mobile station and a base station.

3. The method of claim 1 , wherein the handover situation comprises an inter-radio-access-technology dual transfer mode handover.

4. An apparatus, comprising:

an initiation unit configured to detect or establish a circuit-switched call;

an establishment unit configured to establish, over a cellular network, a voice-over-internet-protocol standby call in parallel to the circuit-switched call, wherein the voice-over-internet-protocol standby call is a standby, virtual call that includes an established end-to-end signaling path but utilizes no physical radio resources, wherein the end-to-end signaling path is ready for use after a cellular handover situation to an active voice-over-internet protocol call established using the end-to-end signaling path;

a handover unit configured to detect a cellular handover situation;

when the detected cellular handover situation is to a first type of network:

an activation unit activates the voice-over-internet-protocol standby call to become the active voice-over-internet protocol call, and

a release unit releases resources of the circuit-switched call, when the active voice-over-internet protocol call is established; and

when the detected cellular handover situation is to a second type of network, wherein the second type of network is different from the first type of network:

the handover unit performs the cellular handover using dual transfer mode signaling, and

a voice-over-internet protocol standby unit maintains the voice-over-internet protocol standby call in a standby state.

5. The apparatus of claim 4 , wherein the release of the resources of the circuit-switched call comprises termination of the circuit-switched call.

6. The apparatus of claim 4 , wherein the apparatus is comprised in a mobile station, and the apparatus is configured to establish the circuit-switched call.

7. The apparatus of claim 4 , wherein the apparatus is comprised in a base station, and the base station is configured to detect the circuit-switched call.

8. The apparatus of claim 4 , wherein the handover situation comprises an inter-radio-access-technology dual transfer mode handover.

9. A computer program product comprising a non-transitory computer-readable medium encoded with instructions that, when executed by at least one processor, perform at least the following:

detecting or establishing a circuit-switched call over a cellular network;

establishing, over a cellular network, a voice-over-internet-protocol standby call in parallel to the circuit-switched call, wherein the voice-over-internet-protocol standby call is a standby, virtual call that includes an established end-to-end signaling path but utilizes no physical radio resources, wherein the end-to-end signaling path is ready for use after a cellular handover situation to an active voice-over-internet protocol call established using the end-to-end signaling path;

detecting the cellular handover situation;

when the detected cellular handover situation is to a first type of network:

activating the voice-over-internet-protocol standby call to become the active voice-over-internet protocol call, and

releasing resources of the circuit-switched call, when the active voice-over-internet protocol call is established; and

when the detected cellular handover situation is to a second type of network, wherein the second type of network is different from the first type of network:

performing the cellular handover using dual transfer mode signaling, and

maintaining the voice-over-internet protocol standby call in a standby state.

10. The computer program product of claim 9 , wherein the releasing the resources of the circuit-switched call comprises terminating the circuit-switched call.

11. The computer program product of claim 9 , wherein the method is performed by a mobile station, and the method comprises the establishing the circuit-switched call.

12. The computer program product of claim 9 , wherein the method is performed by a base station, and the method comprises the detecting the circuit-switched call.

13. The computer program product of claim 9 , wherein the handover situation comprises an inter-radio-access-technology dual transfer mode handover.

14. The method of claim 1 , and wherein the voice-over-internet-protocol standby call is identified by an indicator to establish the voice-over-internet-protocol standby call.