[Serusers] Via-received mystery

HI Everyone, I have two UA(Cisco 7960) behind a Linksys firewall and a SER in the public domain. Both the phones can make calls to each other and everything works great.But the mystery is when the invite or register reaches the SER, the SER never recognizes the call as coming behind a NAT, cause the condition in the ser.conf that via!=received fails because I cant see any received parameter in the via field. Hence I guess setflag(6)(mark as nated) also fails. But the whole thing is my calls are working fine. Is there something I am missing altogether? Hows is the whole thing working? Thanks, Hitesh.