[Users] Response to Nat problem

Hello! We've a problem with a provisional response. The invite is sent through NAT , and the response will be sent to incorrect port. The invite is sent from 193.77.152.140:53559 to ser, (it is a natted firewall. It not insert Via header, don't understand sip) The topmost via is: Via: SIP/2.0/UDP 10.10.1.2:5080;branch=z9hG4bK-a47f2525b20ab1a953a7c74174551819;received=193.77.152.140 The server recognize, that it is received from 193.77.152.140, but the reply will be sent to 193.77.152.140:5080 according to the RFC 3261 the port used in the response is , what present in the via header. How can i force that the response will be sent to the source port (52559)? Thanks, Tamas