Thank you David, for response.
I found out that the softphone that operators use(Zoiper) didn’t receive any early media, there is complete silence until the call is picked up and until outbound message is finished. Then I tried myself on my softphone(MicroSIP) and I did receive a ringback tone, although after client picks up the call, ringback tone stops and there comes silence again(during which the message is played),
so result is that Zoiper fails completely in this case.
MicroSIP kind of works but not completely.
Furthermore, I tried to add an option “r” in Asterisk Trunk Dial Options - TtrA(custom/outbound message). And I(as an operator) did receive the ringback as I wanted. I’m not sure if this option will cause any other inconveniences as I’ve read here - tm1000 , but I think it’s worth trying.