Most accidental dialings come from hotels. In a typical PBX system, you dial 9 to get an outside line, then dial 1 for long distance, then your number. However, some people get mixed up, and end up calling 911.
The problem gets worse if they quickly hang up... because when a 911 operator receives a call from a hotel, they see the name and address - but not the room number. So the operator calls the hotel, but the front desk agent doesn't know who called 911. In most jurisdictions, a 911 hang-up requires sending a police car out to investigate.
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