Some towns & cities have their gas & water meters tied in at the D-marc or net-pop as well.
It wouldn't be the burg or fire alarm,(at least if it's properly tied in with line seizure) as that would cut you off.
Try disconnecting everything at the D-marc except the line to the system and then replace the disconnected lines 1 at a time until it starts again. once it starts again, disconnect it, throw a tone on it and see what the heck it feeds.
Also is the auto privacy mode set for all extensions on the partner?

[This message has been edited by MrGemini (edited January 24, 2004).]