This scheme allows the mobile telephone networks to restrict access in a specific area to registered numbers only and is normally invoked by the Police Incident Commander (although it can be invoked by the Cabinet Office). The emergency services are responsible for registering their key numbers in advance.
ACCOLC is indicated on the SIM card by a number in the range 0 - 15. Ordinary cellphone users have numbers in the range 0 - 9. Higher priority users are allocated numbers 10 to 15 During an emergency, some or all access classes in the range 0 - 9 are disabled. If the overload condition continues, mobiles with access classes level 10, 11, then 12 and so on may also be disabled by the operator.
As ACCOLC can be a frustration to normal network users, it is normally only initiated after careful consideration. In the UK, the authority of a Police "Silver" is required (major incident control is named in three tiers in the UK, gold, silver and bronze, in accordance with the London Emergency Services Liaison Panel, a group responsible for creating best-agreed procedures for dealing with various emergency situations) after consideration with the co-ordinating group.
Not all calling by regular mobiles is prevented. When a user attempts an emergency call (digits are dependent upon the country), the mobile will use a different access class in the 10-15 range, and not the access class in the 0 to 9 range. This smart technology in the phone will ensure that priority is given to mobiles who are attempting an emergency call.