Roaming lte emergency call with call back number retrieval

A computer-implemented method, comprising:

• receiving an anonymous Session Initiation Protocol (SIP) invite message at a Proxy Call Session Control Function (P-CSCF) of a Visited Public Land Mobile Network (VPLMN) for a Long-Term Evolution (LTE) emergency call initiated from a wireless user device that is roaming on the VPLMN, the anonymous SIP invite message lacking a Mobile Station International Subscriber Directory Number (MSISDN) of the wireless user device and including an International Mobile Equipment Identity (IMEI) of the wireless user device;
• sending the anonymous SIP invite message from the P-CSCF to an Emergency Call Session Control Function (E-CSCF) of the VPLMN for the E-CSCF to forward the anonymous SIP invite message to a Location Retrieval Function (LRF) in a Gateway Mobile Location Centre (GMLC) of the VPLMN;
• obtaining a MSISDN of the wireless user device that matches the IMEI in the anonymous SIP invite message via the LRF from a database of the GMLC;
• and
• sending a supplemented SIP invite message that includes the MSISDN from the GMLC to the E-CSCF for the E-CSCF to route the LTE emergency call with the MSISDN to a particular Public Safety Answering Point (PSAP).

One or more computing devices on a Visited Public Land Mobile Network (VPLMN), comprising:

• one or more processors;
• and
• memory having instructions stored therein, the instructions, when executed by the one or more processors, cause the one or more processors to perform acts comprising:
• receiving an anonymous Session Initiation Protocol (SIP) invite message at a Proxy Call Session Control Function (P-CSCF) of the VPLMN for a Long-Term Evolution (LTE) emergency call initiated from a wireless user device that is roaming on the VPLMN, the anonymous SIP invite message lacking a Mobile Station International Subscriber Directory Number (MSISDN) of the wireless user device and including an International Mobile Equipment Identity (IMEI) of the wireless user device;
• sending the anonymous SIP invite message from the P-CSCF to an Emergency Call Session Control Function (E-CSCF) of the VPLMN for the E-CSCF to forward the anonymous SIP invite message to a Location Retrieval Function (LRF) in a Gateway Mobile Location Centre (GMLC) of the VPLMN;
• obtaining a MSISDN of the wireless user device that matches the IMEI in the anonymous SIP invite message via the LRF from a database of the GMLC;
• retrieving a PSAP Uniform Resource Identifier (URI) of a particular Public Safety Answering Point (PSAP) via the LRF of the GMLC;
• and
• sending a supplemented SIP invite message that includes the MSISDN and the PSAP URI from the GMLC to the E-CSCF for the E-CSCF to route the LTE emergency call with the MSISDN to the particular PSAP.