The most common/widely used emergency number is 112. It is an emergency telephone number that can be dialled free of charge from most mobile telephones, and in some countries, fixed telephones, in order to reach emergency services (ambulance, fire and rescue, police). Used throughout Europe.

111 is used in some countries such as Australia.

911 is used in the USA

999 is used in the UK and was the first emergency number to be used in the world, being introduced in London in 1937. The choice of 999 was determined by ease of use on the old rotary handsets - easiest to find in the dark.

On old rotary dial telephones, it made sense to use those digits that required the least dial rotation, such as 1 and 2. It also maximized dialling speed.

More Info: en.wikipedia.org