In an emergency
Emergencies can occur for many reasons, usually unexpectedly. In Australia, emergency assistance is reached by dialling 000 on any telephone. This will put you through to an emergency call centre. If you are using a mobile phone, you can dial 112 and, if any signal can be received from any mobile network (ie not necessarily from your provider), your call will be directed to the emergency call centre. Note, however, that although the telephone companies proudly say they provide coverage for something like 95% of the population, once you leave the main roads and coastal areas, much of Australia is out of reach of mobile phone services.
