Hello Experts.
Why sometime when UE is in 3G we see SIP messages exchanged between UE and network?
Like INVITE in DL?
This is really weird, I know, but this is how it is…
And before that UE did LAU request and accept in 3G.
So Core should be aware that UE is not in 4G anymore, it is in 3G.
Hierarchically IMS stand on of EPC.
But there is no direct messaging / heartbeat between EPC and IMS.
When UE loses LTE connection generally there is some kind of race condition if UE can achieve to send SIP (de)register message over default bearer.
IMS knows UE has gone.
If not IMS is not aware of this event.
Untill a T-ADS query over HSS and MME or a downlink message transaction.