I have the same, so how can we catagorize or solve this problem?
I've been working with Asterisk for over 7 years, but I haven't done an actual dial plan config for 5! I don't know how many of the old ways to lock down the system would still work. I think I still remember enough to understand the various options so I'm not worried about having an insecure setup (whenever I get to it).
What I used to run was solid. I remember seeing attempts at cracking the PBX. My dial plan was built in such a way it prevented random people from gaining access to long distance.
I think joining the Asterisk mail list might be a good option. Failing that, research, research, and study until you drop. It'll be the only to understand how Asterisk is configured and how to change it.
Question: does LinuxMCE use plain old Asterisk, or a bunch of different apps that help administrating an Asterisk system easier? I haven't looked into LinuxMCE's telephony feature set, at all.