My first job out of college I worked in the IT department of a construction company. The company headquarters at the time was based in an old mansion which had belonged to the original company owner. In all the time I worked in that building nothing strange every happened.
After I had been with the company for about 9 months, it was announced that we would be moving to a new building. Being in IT, I had a pretty big hand in the move, tracing cables and ordering upgraded replacements, setting up the plan for getting new user computers online, all that jazz. On of my biggest roles though was getting the new phone system ready. As part of this, I worked with the phone company and, on the day of the move, they severed phone service to the old building and moved it to the new building. When this happened, the phones in the old building went offline.
Cut to 1am the night of the move, and we realize that the company that we ordered fibre cables from ordered the wrong cables. I leave my boss and the level 1 support guy in the new building and swing back to the old place to pick up the old fibre cables. As I'm in the old server room trying to free the old cables from the rats nest we left behind, I think I hear a phone ring somewhere in the building. At the time, I just disregard the thought and keep working. It takes about an hour to get all the cables free and then I head back.
A week after the move, my boss tells me to go back to the old building and do an inventory of all the phones we left behind, something about selling them off cheap. I swing over and start on the bottom floor, going room to room marking each phone I find and the type. I walk out of the old staff lounge (a converted kitchen) and into the IT Department, when I distinctly hear the kitchen phone ring. There was no way I could deny this. I froze, there was no way that phone could be ringing, the phone system was offline and we had no outside lines going to the building. I step out of the IT Department and the ringing stops. I'm more than a little freaked out, but I keep going, the sooner I get done, the sooner I can get out. I head to the 2nd floor and, while I'm in the stairwell, I hear a phone downstairs start to ring again. I freeze again and all I can think is that I want to get out of this building. However, I still had 2 floors to go and would be sent back anyway if I didn't finish.
Going onto the second floor, I open the door and immediately one of the phones starts to ring. At this point, I noped out and ran to my car, sat for about 5 minutes shaking before writing down a rough estimate from what I could remember. I refused to ever go back to that building again after that, even with people with me.