VoLTE – Some Thoughts On Emergency Calls

Even if you have a VoLTE capable phone and use it in a VoLTE capable home network, chances are that when push comes to shove and you have to make an emergency call, it’s not done over VoLTE. Instead, CS-Fallback is used to establish the emergency call in GSM or UMTS. But at some point, network operators will likely retrofit their VoLTE installations to be also able to handle emergency calls. I recently wanted to have a closer look and found an interesting resource on the web that saved me a lot of time.

One way to approach the topic is to have a look in GSMA IR.92 and the references contained there to 3GPP specs in the section on emergency calls. Like with many topics in this area, reference follows reference and you have to be very careful not to loose sight of your original target. Another way of approaching the topic is to have a look at an actual message trace and then take it to the specs from there. While it’s often quite tricky to come across traces that are publicly available I was quite lucky this time when I found a full trace of a VoLTE emergency call over at ShareTechnote. Here are two things I didn’t know before:

  • VoLTE Emergency Calls are not made over the IMS bearer. Instead a separate emergency default bearer is established over which an emergency SIP registration is performed and an emergency SIP Invite is then sent. This way, the network can prioritize all interactions with the network right from the start and ensure that the call gets through even when a cell is in overload.
  • There’s a new feature that didn’t exist in GSM or UMTS emergency calls: At the end of the LTE attach or Tracking Area Update process the network can give the mobile device a list of emergency call numbers and which type they are, such as police, fire department, ambulance, etc. This way, the standard opens the way to reach different emergency call centers from the same location. In GSM and UMTS this is not possible, the Emergency SETUP message always looks the same, independent of the number dialed.

There is lots of more to discover in the trace linked to above, most of which will not surprise you if you know how emergency calls are handled in GSM and UMTS.