SINR is bad.
Friends, don’t confuse with 401 message.
It’s just a normal routine to ‘rebuke’ NEs like the security guys. 
Go from 1 -> 2, 2 -> 3 and so on.
There is your reason 
.
Need to solve interference zone.
Yes, I got 503. In fact multiple 503 failures…
But 5xxx belongs to server failures, right?
Then we should link issue to SINR or server failures?
ABout the failures, they occur in a ring.
Hmm… that is related to server failure.
But if possible check B side too.
B side is clear.
In fact, no drop on B side.
Both UEs were together.
Does it get DL invite in that case?
No, invite on UL only.
All Invite SIPSM on UL.
Check with core guys.
For no INVITE on DL?
Check for 503.