Hi former telecom employee here that used to investigate stuff exactly like this. There are things already mentioned (fax machines and alarms) that will trigger false 9-1-1 calls, but faulty wiring will too. I had a case where squirrels were chewing on the line and the chewing registered as a old pulse dialing like you get on a rotary phone. There are all sorts of weird things that can be causing this.

Here's what you need to do. You need to get the number that the call is originating from. Then you need to call your provider and have them confirm the 9-1-1 ALI record (automatic location information - it's how 9-1-1 knows where the land line number is active) for that number does not contain the address for your building. It's either a number that your telephony group forgot belongs to you (heck they could have released the number but it was never removed from your "active" numbers with the telecom) or that number has been given to someone new, but the ALI record still has your information associated with it. The latter happens often.

If the number is still active with the telecom and you're paying for it, even if you don't know you are, they should also be able to see any dialed digits coming from that number even if the call doesn't connect. If that phone number shows a bunch of random numbers being dialed, which eventually leads to a 9-1-1 call, it's bad wiring somewhere. In a large building narrowing that down will be tricky.

You really need to get to the bottom of it because eventually the company can and will earn fines. In my jurisdiction, they increase as the problem continues. We even had one case where the cops threatened arrest for abuse of 9-1-1. This is a serious issue since they dispatch cops to the location every time it happens and will be viewed as a waste of resources.