Tell me what you mean by "if you dial one of the operators".
There are three "operators". Are they DEFINED as operator positions, or do they simply behave as receptionists in a business environment? If they are true operators, please post any display information that comes across the screens when these kind of calls come in. I'm half expecting you to report a "COVER DID #?" message.
And while there may not be any coverage or Shared System Access buttons, it's possible that all phones are in a Group Ring DGC. Any chance that some weird DID number is pointing to a such a group?
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Any sufficiently advanced technology is indistinguishable from Magix. --Arthur C. Clarke (imbellished).