As network is moving towards LTE, operator have to comply with local authorities. What it means that they need to provide certain types of calls (example 911) even if there is no subscription for that UE in HSS which means means there is need for an emergency APN.
When UE wants to perform emergency attach, it will send an attach request with attach type as Emergency Attach. There is a process at MME which is as follows (dont know if there is any changes to be done at eNodeB)
1. Default Emergency APN should be configured in MME in case where UE does not send APN in attach request and also HSS does not provide it in Subscription data for Emergency attached UE or Authentication is optional. In that case MME provides its default APN and Qos and procced the emergency call.
2. MME rejects the emergency call when APN received from UE is not configured in MME Emergency APN list.
3. MME rejects the emergency call only when HSS sends Authentication failure with cause ‘IMSI unknown’. For all other authentication failure cause MME proceed with the call successfully.