Why was 911 chosen as the emergency phone number?

Many countries' public telephone networks have a single
emergency telephone number, sometimes known as the universal emergency telephone number
or occasionally the emergency services number, that allows a caller to contact local
emergency services  for assistance. The emergency telephone number may differ from
country to country. It is typically a three-digit number so that it can be easily
remembered and dialed quickly. Some countries have a different emergency number for each
of the different emergency services; these often differ only by the last digit. Inside
the European Union, 112 has been introduced as a common emergency call number during the
1990s, and is a well known emergency number in the world today alongside with 911 and
999.

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