Good question, I haven't run into that yet here. I remember that Cablevision has a disclaimer of sorts saying basically that their phone service can be used for alarm reporting but don't rely on it and they are not responsible if you do. I know that my service goes down for maybe 10 minutes at a time every once and awhile, didn't check but I suspect that they are rebooting my modem. Probably why security systems now use cellular as a backup. Fire systems require two dedicated copper lines, primary and secondary also.
-Hal
