I got this question long back from one of 3glteinfo reader. Here is the specific question he asked:
I have one doubt in VoLTE call flow like how we differentiate for MO VoLTE and MT VoLTE call?
Means if we were given two MO/MT logs for VoLTE then how we can identify like which one is MO VoLTE or which one MT VoLTE? Is there any way to identify? if you find the answer, please let me know.
VoLTE MO-Call (Mobile Originating)
This is when a VoLTE call is started on the device under test (DUT).
The precondition is, the UE is already registered with the IMS server.
UE-1, in this case, is the originating UE and UE-2 is the terminating UE.
UE-1 sends SIP INVITE message to UE 2
The first message is the SIP INVITE message. This message MUST have the Direction header UE to Network.
Next, check the SIP Message header field. This shows that the message is INVITE and the sip URI is the called party number (UE-2).
So you will find similar to this
Sip Message = INVITE sip:+email@example.com;user=phone SIP/2.0
The next header field is the From field. This shows the phone number used by the DUT (UE-1) and a unique tag identifier.
The To header field has the phone number of UE-2
UE receives SIP TRYING 100 response
The response to the INVITE message is the network tells UE-1 that it is trying to reach the UE-2
So the IMS network sends the SIP response 100 TRYING.
Later the UE-1 and UE-2 negotiate the codec to use and may agree and start a VoLTE call.
Mobile terminating VoLTE Call
Similar to the first case, UE-1 is IMS registered.
UE-1, in this case, is the terminating UE and UE-2 is the originating UE.
Here is a simple call flow scenario what happens when a VoLTE enabled UE receives an MT call.
The UE receives a Paging message.
UE Receives SIP INVITE Message
The first message the UE receives is the SIP INVITE message. Here are certain things to check:
Direction: NW to UE (Downlink)
From: sip URI of the phone which started the call.
From: "+12345678" <sip:+firstname.lastname@example.org;user=phone>;
The simple way to know a call is Mo or MT is by checking the Direction header field in SIP INVITE message.
If it is an MO call, the Direction header field should be UE to Network. And, for MT call the field must be NW to UE.