23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS
Procedure for determining the list of emergency numbers:
– In addition to the emergency numbers and associated types stored either on the USIM or on the user equipment specified in clause 10.1.1 of TS 22.101 , the UE shall use the stored Local Emergency Numbers List received from the 3GPP network to detect that the number dialled is an emergency number.
– If a UE has a SIM/USIM and has received a list of emergency numbers, and associated types via mobility management procedures as described in TS 24.301  or TS 24.008  when connected to the PLMN using GERAN, UTRAN, E-UTRAN or TS 24.501  when connected to NG-RAN or non-3GPP access connected to 5GC, or, for a UE supporting access to 5GC using non-3GPP access, as specified in Annex L, the UE shall use this list to determine if the dialled digits correspond to an emergency call as long as the UE has determined that the list is valid (see clause 4.1).
– The UE may also determine that the dialled digits and associated types or URN information correspond to an emergency call based on the list of emergency numbers retrieved by one of the following means:
NOTE: How the UE uses two lists (a list of emergency numbers and associated types or URN information received via mobility management procedures and a list of emergency numbers and associated types or URN information retrieved via the WLAN) is a stage 3 decision.
a) via Access Network Query Protocol (ANQP) procedures defined in IEEE Std 802.11 2012  from the associated WLAN AP upon completing authentication with the trusted WLAN access to EPC, with management frame protection enabled.
b) via a previous query to a DNS only when that DNS is in the same country as the UE and is an internal DNS server in the 3GPP network whose address is acquired from the ePDG; or
c) via IKEv2 procedures from ePDG.
– The list of downloaded emergency numbers shall be deleted at country change or, if the particular list is only valid for a PLMN, see clause 10.4.1 of TS 22.101 ), at successful registration with a different PLMN.
For the specific case where the UE has selected to make an emergency call over WLAN access to EPC (hence the UE has detected that the target URI or dialled digits correspond to an emergency sessions), the UE shall use the following procedures:
– The UE shall establish an emergency PDN connection over WLAN and shall perform an IMS emergency registration before sending an IMS emergency session request.
– The UE shall include any available location information in the IMS emergency session request. This is further detailed in TS 23.228 .
– For the media supported during IMS emergency sessions, media codec and format support is specified in TS 26.114 .
– For the support of DRVCC on emergency call on WLAN, see TS 23.237 .
A UE shall not establish an emergency PDN connection over WLAN if the UE initiated a normal (i.e. non-emergency) session and is subsequently informed by the network that the ongoing session is accepted by the network even though it is an emergency session.