Question: What happens when a UE in substate EMM-DEREGISTERED.eCALL-INACTIVE receives a request from upper layers to establish an eCall over IMS?
Option 1: The UE ignores the request and remains in substate EMM-DEREGISTERED.eCALL-INACTIVE.
Option 2: The UE enters state EMM-NULL and powers off.
Option 3: The UE initiates EMM signalling with the network to establish a non-emergency call.
Option 4: The UE enters state EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH and uses relevant EMM and ESM procedures to establish the eCall over IMS at the earliest opportunity.
Explanation: According to the 3GPP TS 24.301 specification, when a UE in substate EMM-DEREGISTERED.eCALL-INACTIVE receives a request from upper layers to establish an eCall over IMS, it enters state EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH and uses the relevant EMM and ESM procedures to establish the eCall over IMS at the earliest opportunity. This is because the UE needs to attach to the network to establish the eCall session.