A survey of more than 2,000 adults by BT showed that 4% did not know when to call 999 or 112, or non-emergency numbers such as 101 and 111.
Only around a third knew they could call 101 for a non-emergency police issue.
The number of 999 calls handled by BT has increased from 25 million in 2000 to 33 million last year, with most made from a mobile phone.
Most of those polled were not aware that smartphones can provide the exact location of a 999 caller.
Geoff Hickman, of BT, said: "It's clear from the survey that not everyone knows when to call the emergency number, and when to call the range of non-emergency numbers.
"With nearly 80% of people now using a smartphone, we have a lot of people carrying potential life savers in their pockets.
"Using a smartphone's location service, our call handlers can now pinpoint a 999 caller to within three metres in seconds.
"This could mean, for example, being able to tell which side of the motorway the call has come from, helping an ambulance get to a scene 10 minutes earlier, which is potentially life saving."
The report was published to mark 999 Day, an annual celebration of the work of the emergency services.
Six BT centres handle all the UK's 999 calls, passing them on to the relevant emergency service.