Help! Police! Fire! Doctor!
So you're in the UK and you need one of the emergency services. You pick up the phone and dial. But what number. Well in the UK you dial 9 9 and then 9 again. Unlike in the states where you dial 911. You'll be passed immediately through to an operator who will ask you which service you require.
Each of these services respond as fast as they can. Many will contact the others if they might also be needed. For instance a fire, the police and ambulance will often arrive as well encase there are those that need medical attention and encase the fire is an act of criminality.
Calling 999 is completely free to call, in fact most mobile phone providers allow this number to be dialed despite a lack of credit on a phone.