OK, I see why. You call flow is like this:
Customer → Kamalio LB → SEMS
So because SEMS is not receiving directly from customers, you need to use the X Yeti Auth field. Kamalio will pass the customer’s IP and you can match it using the X Yeti Auth field. I don’t remember how again (might need to install an LB to be remember)
refer to: SIP INVITE via LoadBalaner with Require Incoming Auth Enabled - #6 by rabbani