Fixed Dialing Number is a service mode of a GSM phone's Subscriber Identity Module

Subscriber Identity Module

A subscriber identity module or subscriber identification module is an integrated circuit that securely stores the International Mobile Subscriber Identity and the related key used to identify and authenticate subscriber on mobile telephony devices .A SIM is held on a removable SIM card, which...

 (SIM) card. Numbers are added to the FDN list, and when activated, FDN restricts outgoing calls to only those numbers listed, or to numbers with certain prefixes. A notable exception is that emergency calls to 000, 112, 9-1-1, 999

Emergency telephone number

Many countries' public telephone networks have a single emergency telephone number, sometimes known as the universal emergency telephone number or occasionally the emergency services number, that allows a caller to contact local emergency services for assistance. The emergency telephone number may...

 and the like are exempt (and typically require no SIM or even an active service contract). Incoming calls are not affected.

Not all SIM cards support this feature. PIN2 must be entered beforehand, which is unique to each SIM card and initially provided by the network operator. Doing so prevents unauthorized users from changing or disabling the FDN list. This code is separate from the main PIN1 number which, when set, can lock and unlock the SIM card for both inbound and outbound calling.

One practical application of FDN is for parent

Parent

A parent is a caretaker of the offspring in their own species. In humans, a parent is of a child . Children can have one or more parents, but they must have two biological parents. Biological parents consist of the male who sired the child and the female who gave birth to the child...

s to restrict the phone numbers to which their children can dial. For example, the list may give the child permission to call only their mother and father, a relative, or a family friend.