I have finally discovered why the audio on the ip phone doesn't work. My system configuration: - Windows 2000 Professional with SP4 - 3CX v.3 - 3 Ethernet interface with t following IP address 172.31.253.1 192.168.0.1 10.0.0.1 The machine is behind a firewall, with all the necessary port NATTED to the IP 172.31.253.1 of the machine that run the PBX software. All the phone and the VOIP service can register without error, but when someone call my number I can hear the caller's voice, but on the other side the person can't hear mine. I have find out that disabling the 2 ethernet with the IP 192.168.0.1 and 10.0.0.1 solve this problem. In fact seems that the server listen on all the 3 interfaces, but the port necessary are natted only on the IP 172.31.253.1. So when the server use IP other than 172.31.253.1 to make calls, the communication doesn't work. How can I specify manually the address on wich the server have to listen? I need all the 3 interface, so I can't leave disabled the other 2. Thanks.