ACCOLC (Access Overload Control) was a procedure in the United Kingdom for restricting mobile telephone usage in the event of emergencies.
This actively prevents unnecessary usage from congesting the cell networks, thus allowing emergency services personnel priority for communications.
Mobile networks can become overwhelmed by a high concentration of calls that often occur immediately after a major incident.
Reliable access to the mobile networks, even during times when an exceptionally large number of calls are being made, is achieved by installing a special SIM (subscriber identity module) card in the telephone handset.
For special case mobile customers – e.g. emergency services, government officials, civil defence, etc.