Program 51-11 gives the MAC address of all Netlinked sites. From your post I can tell you are not the vendor or a certified tech so I will try and help what I can. The signaling IP is NOT the same as the Media GW IP. Over fiber meaning what network wise? Do you have a VPN setup or do you have dedicated fiber for just your network? There are many things it can be. Generally in a VPN setup you have ports blocked at a certain range that causes this or you have something that is not lined up on your UDP ports (the media gateway IP) that is making you lose talk paths. I heavily suggest getting the vendor out and verify the network settings, pbx settings and the network. Just remember the media channels do a round robin so ALL the media cards have to be the same size AND same revision. All processors have to be on the same software and MEM cards. The ONLY thing that the primary site can have the inmail.