Telecommunications network non-establishment response

1. A telecommunications network, comprising:

a core network device communicatively connectable with user equipment, wherein the core network device is configured to:

receive session-progress information of a communication session with the user equipment;

subsequently, receive a session-failure indication;

determine action data based at least in part on the received session-failure indication, the determining including retrieving the action data from a data source mapping multiple, different instances of action data to multiple, different instances of session-failure indications and multiple, different access network types, with at least two different instances of session-failure indications mapping to two different access network types; and

in response to the action data indicating a retry, re-initiate the communication session via an access network indicated in the action data.

2. The telecommunications network of claim 1 , wherein the core network device includes a telephony application server (TAS) or a service centralization and continuity application server (SCCAS).

3. The telecommunications network of claim 1 , wherein the core network device is further configured to receive a session-commencing indication and provide the session-failure indication in response to expiration of a timeout period defined with reference to a time of receipt of the session-commencing indication.

4. The telecommunications network of claim 1 , wherein the core network device is further configured to provide the session-failure indication in response to expiration of a timeout period defined with reference to a time of receipt of the session-progress information.

5. The telecommunications network of claim 1 , further including a call session control function (CSCF) server communicatively connected with the core network device and configured to provide the session-failure indication in response to a failure to set up a dedicated bearer channel for the user equipment.

6. The telecommunications network of claim 1 , wherein the core network device is further configured to initiate a communication session with the user equipment.

7. The telecommunications network of claim 1 , wherein the telecommunications network further includes a first access network of a first type and a second access network of a second, different type and the core network device is further configured to initiate the communication session via the first access network and, in response to a selected error code in the received session-failure indication, provide the action data including an indication of the second access network.

8. The telecommunications network of claim 7 , wherein the first type is a packet-switched (PS) network type and the second type is a circuit-switched (CS) network type.

9. The telecommunications network of claim 1 , wherein the telecommunications network further includes a first access network of a first type and the core network device is further configured to initiate the communication session via the first access network and, in response to a selected error code in the received session-failure indication, provide the action data including an indication of the first access network.

10. The telecommunications network of claim 1 , wherein the telecommunications network further includes a first access network of a first type and a second access network of a second, different type and the core network device is further configured to:

initiate the communication session via the first access network;

in response to a selected error code in the received session-failure indication, provide the action data further including an indication of a relative order of the first access network and the second access network;

carry out the re-initiating via the access network indicated first in the relative order in the action data; and

in response to receipt of a selected re-initiation error code, re-initiate the communication session via the access network indicated second in the relative order in the action data.

11. The telecommunications network of claim 1 , wherein the core network device is further configured to determine the action data by retrieving the action data from the data source, and wherein the data source maps the action data to other information, and the other information includes at least one of an access-network type of the communication session, or a unique identifier of a radio carrying the communication session.

12. The telecommunications network of claim 1 , wherein the core network device is further configured to, in response to the action data indicating no retry, transmit a session-failure indication to the user equipment.

13. The telecommunications network of claim 1 , wherein the core network device is further configured to, before receiving the session-progress information:

receive a pre-progress session-failure indication;

determine pre-progress action data based at least in part on the received session-failure indication; and

in response to the pre-progress action data indicating a retry, re-initiate the communication session via an access network indicated in the pre-progress action data.

14. The telecommunications network of claim 13 , wherein the core network device is further configured to determine the pre-progress action data by retrieving the pre-progress action data from the data source.

15. A method comprising:

receiving, during a pre-establishment phase of a communication session, a session-failure indication;

determining, by a processing unit, action data based at least in part on the received session-failure indication, the determining including retrieving the action data from a data source mapping multiple, different instances of action data to multiple, different instances of session-failure indications and multiple, different access network types, with at least two different instances of session-failure indications mapping to two different access network types; and

in response to the action data indicating a retry, re-initiating the communication session via an access network indicated in the action data.

16. The method of claim 15 , wherein the data source further maps the multiple, different instances of action data to other information, and the other information includes at least one of an access-network type of the communication session, or a unique identifier of a radio carrying the communication session.

17. The method of claim 15 , further including receiving session-progress information of the communication session and, subsequently, performing the receiving session-failure, determining, and re-initiating.

18. The method of claim 17 , further including, before receiving the session-progress information:

receiving, by the core network device, a pre-progress session-failure indication;

determining, by the core network device, pre-progress action data based at least in part on the received pre-progress session-failure indication; and

in response to the pre-progress action data indicating a retry, re-initiating, by the core network device, the communication session via an access network indicated in the action data.

19. The method of claim 18 , wherein the determining the pre-progress action data includes retrieving, by the core network device, the pre-progress action data from the data source.

20. User equipment comprising:

a processing unit;

one or more radios configured to selectively communicate via a first access network and via a second access network; and

one or more components communicatively coupled to the radio and operated by the processing unit to perform operations including:

receiving an invitation message of a communication session with another device via the first access network, wherein a pre-alerting phase follows or commences with the receipt of the invitation message and precedes transmission of an alerting-progress message;

transmitting, in response to the received invitation message and during the pre-alerting phase, a preconditions message;

subsequently, detecting, during the pre-alerting phase, a failure condition preventing establishment of the communication session;

transmitting, in response to the detection of the failure condition, an error code indicative of the failure condition; and

subsequently, receiving a second invitation message of the communication session via the second access network

wherein an access network type of the second access network is selected by a core network based on the error code.