When one call 911, how is the call automatically local?

Question

Hypothetically speaking, if someone is living in New York and they call 911, how does the call not go through a hospital in Los Angeles or other city from another state? How does the call automatically transfer to a local hospital in the area one lives in?

In: Technology

Answer 1

Going back to the old days of simple telephone systems, when you dial a number, your phone is connected to a local exchange. The exchange receives the numbers in the order they are dialled, and routes the call accordingly. In the NANP (North American Numbering Plan), a “normal” number is 7 digits, with the first group of three digits identifying the exchange and the second group of 4 identifying the individual line from that exchange. When the plan was created, it was felt that no local area would need 1000 local exchanges, so it was possible to reserve certain numbers that would never be allocated to normal exchanges for specific purposes.

555, for example, is generally dedicated for dramatic use. It is never allocated as a real number, so film and TV can use numbers starting 555 for fiction, and there will never be a real phone with that number. 0 was reserved for the operator and 1 was reserved for accessing long distance trunk dialling, so no exchange starts with a 0 or a 1. The numbers x11 were all reserved for specific purposes in the local area. For example 611 was used for reporting faults with the phone system. If a number in this range is dialled, the local exchange will route it, locally, to the relevant service. When it was decided to adopt a universal emergency number, 911 had not been allocated for any other purpose, so it was free for this use.

Back in the day, if you dialled 911, it would route you from your local phone exchange to the local emergency services operator, based on the exchange to which the phone you are calling from is connected. Obviously with cellular phones things are a little more complicated, but basically the same principal applies: if you dial 911 from a cell phone, it routes you to the emergency operator that is local to whichever cell phone tower your phone is connecting to.