Personally, I feel the conference bridge is a potential issue... Not security, but privacy (anyone knowing the code listen in at any time), but also conferences do not require an internal party, so there is a potential of fraudulent or criminal activity occurring on your conference bridge. I have setup a LOT of these and I have never had any issues arise, but it is something that sticks in the back of my mind as a very "odd" way for Mitel to set this up.