How user get notified for MT VoLTE call?

Hello VoLTE Experts.

How user get notified for MT VoLTE call?

If it’s IMS registered.

Is it by paging?

Yes, it is by paging.

What that paging indicates?

Any special message?

It will send you depending, stmsi, tai between some others.

Then the UE will reply with a initial UE message with a service request embed.

Ok, invite message go from A to B party or how is the flow?

If it’s MT, the paging is sent to the B party.

And the B party replies with the initial UE message with RRC code MT access.

Initial UE message is at backend, what about SIP messages exchange?

The B party receive the invite sent from a party after the signalling it’s stablished.

Then the B party replies with a 100 trying, I guess.

And then all SIP signalling continues.

I think paging from B to A.

Who’s A and who’s B for you?

Who starts the call?

B calling party, A receiving for MT call.

Ok, so B is calling A, then A receives the paging.

Yes, that initial UE message not clear to me…

Can you please elaborate more?


  • Find where is UE A;
  • Then eNodeB sends paging message;
  • UE A receives the paging and replies with a initial UE message (S1AP);
  • And inside this message we have a service request (NAS-EPS) message.

So that is how the UE establish the channel.
So then it can receive the invite (SIP)

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