The monitoring company--that's the key.  As a property manager, I can tell you that what sometimes happens is that the building owner changes companies, but management doesn't take the extra step of changing (programming) the number that the emergency calls go to.  If calls go to the old company, and it is no longer under contract, the calls go unanswered.  I discovered this the hard way, after several people in a building I managed reported that they had summoned help from inside the cab, but had gotten no response.  It's similar to 911.  When a person changes phone carriers, the carrier has to notify the local 911 system of the change.