As specified in the TR 23.749 (Study on S8HR architecture for VoLTE), for calls to a non detectable emergency call, the P-CSCF may return the SIP response 380 "Alternative service".
Referring on the RFC3261 (SIP), the SIP response 380 should include the alternatives services in the message body.
Based on this content, the UE may either retry the call in CS domain (CSFB) or retry a VoLTE call to an alternatives local P-CSCF (unauthorized emergency call in VPLMN).
However, in the last RFC3261 I have (December 7th 2015) the message body format within the response 380 is not defined and is subject to future standardization.
May someone have more information about the format of the message bodies with the SIP result 380?