And here is a short piece of log from the server side:
...
nua(): refersh session after 62 seconds (in [55..65])...
send INVITE ...
rcv OK...
send ACK...
rcv BYE...

I see now that sdp for natted client has additional lines in OK response compared to client with public ip.
Session-Expires: 120;refresher=uas    
Min-SE: 120

How come that they differs? And how do I resolve this situation? Should client handle these refresher messages normally?

Best regards, Robert.