In Australia our emergency phone number is 000, but prior to 1969 Australia did not have a nation number for emergency services. Each service fire, ambo and police each had many different numbers.
In 1961 the postmaster general introduced the Triple Zero number in major populated centres. Near the end of the 1980 it was extended to a nationwide coverage.
Triple Zero was chosen for several reasons, it suited the dialling system for the most remote automatic exchanges like in outback Queensland such communities used the 0 to select an automatic trunk line to a centre and in the most remote communities two 0s had to be used to reach a main centre; thus dialling 000 was considered best also it was felt that it would be easy to dial 000 in the dark.
You can call 000 from a mobile phone even if the phone is locked or in Australia you can also call 112 from a mobile phone to reach an operator who will connect you to the right emergency service.
911 was at one point considered as a potential emergency number, though existing phone numbers starting with 911 made this unfeasible. However, if a person was to call 911 the call would be re-routed to 000. Since many young people have heard of 911 which is the emergency number in the USA and Canada.
Here in Australia we also have the 131444 phone number for police but that is not for emergencies it is for other police matters it is also a nationwide number, I think.