What is FDN in Samsung Phones?

FDN (Fixed Dialling Number) or FDM (Fixed Dialling Mode) is a service mode of GSM phone's Subscriber Identity Module (SIM) card feature that allows the phone to be "locked" so that it can only dial certain numbers, or numbers with certain prefixes.Incoming calls are not affected by FDN service. 

(Representation of the feature in the settings menu)

Know more about FDN : 

● A PIN code (numeric password) protects the allowed list of numbers from being changed by any person who does not know the PIN code.

● It is a network dependent feature, please contact your service provider.

● This feature is usually found in the settings menu of a Smart phone as shown above.