Hi, Dawood. Thank you for your reply.
Yes, that's correct: the setup you've described is exactly the same as our demo setup. We used an asterisk as a SBC between external SIP trunk and our system.
Now you've dispelled my doubts concerning SIP authetication. We'll consider enabling TLS encryption between CCtr system and Session Border Controller (in our case Asterisk). Thanks a lot for your help.
BR,
Sergey.