the door lock is combined with the phone system being online?
That is probably the most ... fun, setup I have ever heard of
So if thieves are coming to rob the place then all they have to do is cause a power outage and the doors are open. ;-) not sure who came up with that but I would have refused to install it this way.
As for the reboot, that can have a million reasons. Power supply going down slowly, config issue, some firmware issue that causes it when a certain action happens.
Run the Monitor and look what happens just before the incident. Check also your config and make sure there are no issues with duplicated names etc. if you run embedded voicemail make sure it is not full for some users and if so have them delete the messages. That is all however not a guarantee that this will fix it but always a good start.
Joe
FHandw, ACSS, ACIS