I have 3 PBX's all running PBXIAF. What is going on is PBXA is at our main site with a fiber connection and generator backup and all our inbound lines for our 3 office locations come into PBXA. I have sip trunks setup to pass the calls for PBXB and PBXC down to other branches. What i want to do is if PBXB's internet does down that the system senses this and routes all the calls over to PBXC and vice versa. Right now if the PBXB location does down I have to go into the PBX and switch the inbounds to point to PBXC manually and once it come back up change it back. I didnt know if there was an Automatic way for this to happen???? 
Phone System 