999 is the United Kingdom's and Ireland's emergency telephone number and Poland's medical emergency number. They are all used alongside the EU standard 112.
999 is also the emergency telephone number in some Commonwealth countries including  Bangladesh, Botswana, Ghana, Malaysia and Singapore (the rest use a variety of numbers, including 9-1-1). It is also used in the United Arab Emirates, Hong Kong, Macau, Bahrain and Qatar.

# Hong Kong
The 999 was introduced to Hong Kong during British rule and continues to be used following the handover in 1997.

# Malaysia
The 999 emergency services in Malaysia is manned by about 138 telephonists from Telekom Malaysia. On-going upgrading works are taking place to introduce the Computer-Telephony Integration (CTI) for hospital exchanges, digital mapping to track the callers' locations and Computer Assisted Despatching (CAD) for online connectivity among the agencies providing the emergency services in the country. All calls to the number are made free of charge.

# Poland
The 112 emergency number is an all-service number in Poland like in the other EU states, but old numbers that were traditionally designated for emergencies are still in use parallel to 112. Those are:
- 999 for medical emergency
- 998 for fire emergency
- 997 for Police

# United Kingdom
The 999 service was introduced on 30 June 1937 in the London area. 999 was chosen because of the need for the code to be able to be dialled from A/B button public telephones. The telephone dial (GPO Dial No 11) used with these coin-boxes allowed the digit '0' to be dialled without inserting any money, and it was very easy to adapt the dial to dial '9' without inserting money. All other digits from 2 to 8 were in use somewhere in the UK as the initial digits for subscribers' telephone numbers and hence could not easily be used. Had any other digits been used, other digits between that one and the already free '0' would also have been able to be dialled free of charge. No other telephone numbers existed using combinations of the digits '9' and '0' (other than one in Woolwich) therefore there would be no unauthorised 'free' calls. Thus the easy conversion of coin-box dial was the deciding factor and the fact that 999 was not used anywhere, other than for accessing the occasional 'position 9' of an Engineering Test Desk in the telephone exchange. Numbers beginning with 1 were excluded for other technical reasons - for example, 111 could be dialled by accident by wires making contact. Access to the emergency service is provided for the hearing impaired via Textphone and use of the national 'typetalk' relay service. The number is 18000, having previously been 0800 112999.
Since the introduction of mobile phones, the choice of the number 999 has become a particular problem for UK emergency services, as same-digit sequences are most likely dialled by accident due to vibrations and other objects colliding with a keypad. This problem is less of a concern with emergency numbers that use two different digits (e.g., 112, 911).
The pan-European 112 code was introduced in the UK by BT in December 1992, with little publicity. It connects to existing 999 circuits. The GSM standard mandates that the user of a GSM phone can dial 112 without unlocking the keypad, a feature that can save time in emergencies but that also causes some accidental calls. However, a valid SIM card is required to make a 999/112 emergency call in the UK.
On 6 October 1998, BT introduced a new system whereby all the information about the location of the calling telephone was transmitted electronically to the relevant service rather than having to read it out (with the possibility of errors). This system is called EISEC (Enhanced Information Service for Emergency Calls).
On 30 December, 2006, West Midlands Ambulance Service gave Christmas 2006 examples of inappropriate uses of 999 during the festive period, including: a man who could not find his trousers; a man who "couldn't walk from too much dancing"; a man with a finger injury he had sustained two days earlier; and an 18-year-old man who had a toothache.
It has been reported that on some networks in the UK, and in Ireland dialing 9-1-1 will forward you to the emergency line as well. Despite that 911 is not the official number in those locations and can not be relied upon in case of emergency.

## 999 services
In the UK it is an all-service number, meaning that it should be called in any situations where state-run emergency services are needed. The three main and best-known services are police, fire & rescue services and ambulance/paramedics. Other available services  include coastguards, mountain rescue and cave rescue (where locally relevant). Some situations such as a major car accident or a terrorist attack (including nuclear, biological, or chemical attacks) will require multiple and/or specialist services but the first point of call for reporting such incidents from the general public is still the 999 system. In some situations there will be specific instructions on nearby signs to notify some other authority of an emergency before calling 999. For example there are notices on bridges carrying railways over roads telling people that, if they see a road vehicle striking the bridge, they should call the railway authority (on a given number) first and then call 999 to inform the police.
In the UK, the number is operated by BT, Cable & Wireless, Kingston Communications and Global Crossing.  These organisations forward calls to the appropriate emergency service for the location and incident; all calls to the number are made free of charge. The operation of 999 is coordinated by the 999 liaison committee.
Since May 2006 a new non-emergency telephone number 101 has been available, initially in Hampshire, and then in Northumbria, Cardiff, South Yorkshire, and 'Leicester and Rutland' for calls to the police that did not require an immediate police response. It was planned to be rolled out in the summer of 2008, but funding was pulled by the Home Office during 2007, causing some of the 101 lines to close.