Single radio voice call continuity for emergency callback or click-to-dial sessions

1. An apparatus comprising:

at least one processor; and

at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to at least:

receive a message inviting a user equipment to participate in an emergency communication session with an originating node, the user equipment being registered with an Internet Protocol Multimedia Subsystem (IMS), the originating node comprising a public safety answering point inviting the user equipment to participate in an emergency callback session, or an application server inviting the user equipment to participate in an emergency click-to-dial session, wherein another message inviting the user equipment to participate in the emergency communication session is received in response to an inadvertent release of the emergency communication session;

identify a type of registration of the user equipment with the IMS;

select a transfer function of the IMS to anchor the emergency communication session based on the identified type of registration of the user equipment with the IMS, the selected transfer function comprising a domain transfer function or an emergency access transfer function of the IMS; and

prepare the message for transmission to the selected transfer function to thereby enable the selected transfer function to anchor the emergency communication session and further communicate with one or more other components of the IMS to establish the emergency communication session with the user equipment.

2. The apparatus of claim 1 , wherein being configured to cause the apparatus to select a transfer function includes being configured to cause the apparatus to select the domain transfer function in an instance in which the user equipment has a normal, non-emergency registration with the IMS.

3. The apparatus of claim 1 , wherein being configured to cause the apparatus to select a transfer function includes being configured to cause the apparatus to select the emergency access transfer function in an instance in which the user equipment has an emergency registration with the IMS.

4. The apparatus of claim 3 , wherein being configured to cause the apparatus to prepare the message for transmission includes being configured to cause the apparatus to prepare the message for transmission to the emergency access transfer function based on an address of the emergency access transfer function, and

wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to further learn the address of the emergency access transfer function during an emergency registration of the user equipment with the IMS, being configured to cause the apparatus to learn the address including being configured to cause the apparatus to:

receive an emergency registration request from the emergency access transfer function or an emergency call session control function in communication therewith, the emergency access transfer function or emergency call session control function having previously been added to a session path such that the message is routed via the emergency access transfer function or emergency call session control function.

5. A method comprising:

receiving a message inviting a user equipment to participate in an emergency communication session with an originating node, the user equipment being registered with an Internet Protocol Multimedia Subsystem (IMS), the originating node comprising a public safety answering point inviting the user equipment to participate in an emergency callback session, or an application server inviting the user equipment to participate in an emergency click-to-dial session, wherein another message inviting the user equipment to participate in the emergency communication session is received in response to an inadvertent release of the emergency communication session;

identifying a type of registration of the user equipment with the IMS;

selecting a transfer function of the IMS to anchor the emergency communication session based on the identified type of Registration of the user equipment with the IMS, the selected transfer function comprising a domain transfer function or an emergency access transfer function of the IMS; and

preparing the message for transmission to the selected transfer function to thereby enable the selected transfer function to anchor the emergency communication session and further communicate with one or more other components of the 1 MS to establish the emergency communication session with the user equipment.

6. The method of claim 5 , wherein selecting a transfer function comprises selecting the domain transfer function in an instance in which the user equipment has a normal, non-emergency registration with the IMS.

7. The method of claim 5 , wherein selecting a transfer function comprises selecting the emergency access transfer function in an instance in which the user equipment has an emergency registration with the IMS.

8. The method of claim 7 , wherein preparing the message for transmission comprises preparing the message for transmission to the emergency access transfer function based on an address of the emergency access transfer function, and

wherein the method further comprises learning the address of the emergency access transfer function during an emergency registration of the user equipment with the IMS, learning the address including:

receiving an emergency registration request from the emergency access transfer function or an emergency call session control function in communication therewith, the emergency access transfer function or emergency call session control function having previously been added to a session path such that the message is routed via the emergency access transfer function or emergency call session control function.

9. A non-transitory computer-readable storage medium having computer-readable program code portions stored therein, the non-transitory computer-readable storage medium and computer-readable program code portions being configured to, with at least one processor, cause an apparatus to at least:

receive a message inviting a user equipment to participate in an emergency communication session with an originating node, the user equipment being registered with an Internet Protocol Multimedia Subsystem (IMS), the originating node comprising a public safety answering point inviting the user equipment to participate in an emergency callback session, or an application server inviting the user equipment to participate in an emergency click-to-dial session, wherein another message inviting the user equipment to participate in the emergency communication session is received in response to an inadvertent release of the emergency communication session;

identify a type of registration of the user equipment with the IMS;

select a transfer function of the IMS to anchor the emergency communication session based on the identified type of registration of the user equipment with the IMS, the selected transfer function comprising a domain transfer function or an emergency access transfer function of the IMS; and

prepare the message for transmission to the selected transfer function to thereby enable the selected transfer function to anchor the emergency communication session and further communicate with one or more other components of the IMS to establish the emergency communication session with the user equipment.

10. The computer-readable storage medium of claim 9 , wherein being configured to cause the apparatus to select a transfer function includes being configured to cause the apparatus to select the domain transfer function in an instance in which the user equipment has a normal, non-emergency registration with the IMS.

11. The computer-readable storage medium of claim 9 , wherein being configured to cause the apparatus to select a transfer function includes being configured to cause the apparatus to select the emergency access transfer function in an instance in which the user equipment has an emergency registration with the IMS.

12. The computer-readable storage medium of claim 11 , wherein being configured to cause the apparatus to prepare the message for transmission includes being configured to cause the apparatus to prepare the message for transmission to the emergency access transfer function based on an address of the emergency access transfer function, and

wherein the computer-readable storage medium and computer-readable program code portions are further configured to, with the at least one processor, cause the apparatus to further learn the address of the emergency access transfer function during an emergency registration of the user equipment with the IMS, being configured to cause the apparatus to learn the address including being configured to cause the apparatus to:

receive an emergency registration request from the emergency access transfer function or an emergency call session control function in communication therewith, the emergency access transfer function or emergency call session control function having previously been added to a session path such that the message is routed via the emergency access transfer function or emergency call session control function.