[Serusers] Port mapping with Linksys wrt54g

Hello, I am using a Linksys wrt54g router that applies Symetric NAT. When I register with my OpenSer proxy, I set my private IP address to Via, Call-ID and Contact fields, because I have enabled fix_nated_contact(); from the nathelper module. There is no problem with the registration. I get the OK back and openserctl ul show returns public IP and public port in Contact field. Also, OpenSer knows an IP:port to use to forward me SIP messages, or small UDP packets to keep the mapping on my NAT alive. To do so, I use modparam("nathelper", "natping_interval", 10). But I noticed that after each SIP transaction the router changes the mapping used. It means Register and OK on port for example 10000 on the router's public interface and the next INVITE-OK-ACK through port 10005, a second INVITE, through other port. So the mapping done for my 5060 port change very often and also as soon as the register transaction is finished, it becomes false and openSer can't send me any Sip Message. There is no problem with MSN or Skype because the signalling is done on TCP. Do you think registration using TCP instead of UDP will fixe the issue? Have you ever come across that issue? Can it be solved out differently? Thanks, Paul