Maybe the offices close at midnight.

I do know that some companies have phone systems which automatically change modes at a certain time, and in doing so, may disconnect calls (because if it changes to night mode, or to closed mode, it will direct all calls to a message saying the offices are closed, and will disable all incoming calls to the actual phones, sometimes disconnecting any calls in progress).

I don't know whether NWA uses this kind of system, but I do know that there are such things.