Hi I have build a sip environment one pc A(red hat 9) as gateway , only enable NAT , and install ser with nathelper notebook B get ip from pc A , register to PC A's public IP notebook C is public and register SIP to pc A's public IP +-------------------+ | PC C (Msn) | +-------------------+ | | Internet | +----------------------------------------+ | Gateway / NAT | | Ser with Nathelper | | PC A | +----------------------------------------+ | | Intranet | +-------------------------------------+ | PC B (Kphone 3.14) | +-------------------------------------+ B call C , then C pick up the call , but voice from B to C can work , C's voice can't make it My question is Nathelper this module is now only for Cisco is Sip ... ? and is this module need STUN server ? Now , for ser , how to solve NAT/Firewall 's problem ? or it's service provider's issue ? Thanks Jimmy