I have this scheme: SIP Carrier (external IP 22.203) <  >  (local ip 1.21) mikrotik <  > (local ip 1.21) Kamailio - (local ip 1.15) Asterisk.

Carrier does not know what is Route header. that is why Carrier send ACK directly to 1.15 and fail. 

I change Contact at Kamailio to 1.21 then ACK is coming to Kamailio. 

But i  think it's wrong way...

How can i get ACK from CArrier to Kamalio and route it in right way?

Thanks