[Users] Response to Nat problem

[Cseke Tamas]

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