The number 999 was chosen because, on the old dial phones, it was
considered to be the easiest to dial in DARK (darkness) or smoke.
The first three services are the most COMMON (commonly) used.
The procedure is to dial the emergency number, to tell the OPERATE (operator)
which service is needed and to give the address or location where help is
needed.
Where a specialized service is requires, the call for help is often made
to a branch of a voluntary ORGANIZE (organization) or charity.