I've been doing a bunch of testing here with different systems and found its fairly easy to crash a system even when behind a firewall with port forwards in place. You really need to make sure you block all traffic on those ports except for known IP addresses such as your office. I've been battling one location where they have figured out a way to crash the phone system so bad the voicemail corrupts and will not boot back up (7200s). When i go to login to the system after they crash it, OSDM tells me someone else is logged in from IP address XXXXXXXX but the tech programming logs do not reflect so they are not using OSDM to do it.