[Serusers] Cisco IP phone NAT config

Hello, I have SER in bridging mode with two IPs (private and public). To dial the world, my Cisco IP phones must contact the SER private IP, and the call is then proxyed by SER. All other SIP clients can do it, but the Cisco phones dont.... What should I put in the configuration file? For now I have this: # NAT/Firewall Traversal nat_enable: 1 nat_address: "" voip_control_port: 5060 start_media_port: 8000 end_media_port: 9000 nat_received_processing: 0 ...but I m not really using NAT, because, for the phones, the call just goes to the private SER IP, and they dont know nothing besides that. Does someone have a setup like this with Cisco phones? Thanks Joao