We had a similar situation like Don suggested a while back for an office building where they had a lot of after-hours office break-ins. What we ended up doing was routing the line through a cheap, used KSU (Panasonic KX-T616). The door phone entry panel was just a station on the system. We then used the system's SMDR output to log every outgoing call. At first, it was sent to a serial printer but they got tired of dealing with paper and ink, so we went back and sent the serial data into a PC using a Hyperterminal text file. It was clunky, but it served its purpose. I think we got 999 call records per file, so someone had to start/stop the log file each week.
Today's systems can deliver SMDR information via a LAN port, which would make it much easier to locate the collection device at a more practical location, rather than on-site. I know that Panasonic's KX-TDE100/200 has this ability and since it is recently discontinued, it might be easy to purchase.
Of course, there are so many gadgets out there that can get the SMDR data where it needs to go. That's the easy part.
It didn't take long to determine the culprit. It was the building management company's own office. Their number was routed to an answering service company after hours. People were just claiming they left their card key at home, private cleaning crews, etc. The answering service would just buzz them in.
