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. Mobile phones can be used in countries with different emergency numbers. A traveller visiting a foreign country does not have to know the local emergency numbers, however.

The mobile phone and the SIM card have a preprogrammed list of emergency numbers. When the user tries to set up a call using an emergency number known by a GSM phone, the special emergency call setup takes place. The actual number is not even transmitted into network, but the network redirects the emergency call to the local emergency desk.

Most GSM mobile phones can dial emergency calls even when the phone keyboard is locked, the phone is without a SIM card, or an emergency number is entered instead of the PIN. Most GSM mobile phones have 112, 999 and 911 as pre-programmed emergency numbers that are always available. The SIM card issued by the operator can contain additional country-specific emergency numbers that can be used even when roaming abroad.

The GSM network can also update the list of well-known emergency numbers when the phone registers to it.

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