Short emergency numbers were first introduced before mass international travel. The mess is only now being sorted out.
Historically, emergency numbers differ between networks because phone systems were developed independently, for local circumstances. Note the position of the '9' on each of these dials:
New Zealand (still usable)
Sweden (still usable)
As a result of New Zealand 'ingenuity' in labeling the dial, the number 111 was adopted for emergency services even though--you might say because--the system was modelled on the British system and '999'.
There was no need to standardise until relatively recently. Since the introduction of the first short code in 1937 (999; London) the frequency with which people travel internationally has mushroomed--grown dramatically--and with the growth, brought a practical reason for standardisation. Before the travel boom began in the 60s, a standardised emergency number was hardly important because, well, who would need it?
Between the 60s and the late 90s, standardising, if necessary, would have been very difficult. Since the introduction of short codes (999; 911; 112; 000; 111, etc) for access to the emergency operator(s), networks and dialling plans have been built up around the code and the growing system came to occupy the various 'empty spaces' in the available numbers.
The 90s brought digital switching which greatly assists any changes; there's no longer any need to install heavy mechanical equipment and do lots of very physical wiring inside exchanges.
Europe began the standardisation of emergency access in 1991, and the International Telecommunications Union (ITU, a branch of the UN) has for many years been promoting, recommending and advising globally on the adoption of 112 as a standard number for access to emergency services -- The EU, much of wider Europe, and almostall GSM services have pretty much implemented this (Why not, Singapore? Hmm?). If you dial 112 from a GSM mobile handset, your call will be flagged as emergency before it is sent, received and handled appropriately by the network, irrespective of the local dialling convention.
Many other countries have adopted 112 either in fact, in part (ie: 112 connects to one service), or in principle for future implementation.
In the case of New Zealand, a call to 112 or 999 from a landline reaches a recording telling you the correct number. Due to the popularity of a certain American TV programme, a call to 911 is actually re-routed to the genuine 111 emergency operator.
Wikipedia:A .pdf from the US State Dept:
From the ITU:(PDF) A detailed summary of the whys and wherefores of the worldwide numbering system. Somewhat more interesting than it sounds. Page 27 is quite political, Page 33 discusses the ability of the human mind to remember longer ever-longer numbers.
- Due to inefficient use of available numbering ranges, the UK's numbering plan ran out of capacity when only 3% of the numerically possible numbers was used. (Page 31 in the ITU document)
- Britain was the first country to adopt a short code (999) for emergency calls (London, 1937) because the city had started adopting a very sophisticated electro-mechanical routing system for local phone calls in the 1920s. Sophisticated for the time... the .
- still not universally available in the US, which was generally slow to adopt 911 due to administrative and operational fragmentation. (Introduced in 1968 vs 1937 in the UK) is
Reference Image for rotary dials: Everyone else's '9' is here:
(France used '17' for police, '15' for Fire.)