Fixed Dialing Number
Fixed Dialing Number (FDN) is a service mode of a GSM phone's Subscriber Identity Module (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, 911, 999 and the like are exempt (and typically require no SIM or even an active service contract). Incoming calls are not affected by FDN mode.
Not all SIM cards have this feature. PIN2 must be entered beforehand or when applying such settings, which is unique to each card and initially provided by the network operator. Doing so prevents unauthorized users disabling the FDN list. This code is separate from the main PIN1 number which can lock and unlock the SIM card for both inbound and outbound calling.
One practical application of FDN is for parents to restrict the phone numbers to which their children can dial. For example, the list may give the child permission to call only their relatives or friends. Companies also use this to restrict employees from using company phones for personal use.