Fixed dialing number
Fixed dialing number ( FDN ) or German number restriction is a feature of the SIM card ( Subscriber Identity Module ) of GSM telephones.
function
The phone numbers or number ranges are saved in the FDN list. If you activate the feature, then outgoing calls are limited to these in the FDN list. Emergency numbers are not affected and can be dialed. Not all SIM cards support this feature. PIN2 is required for activation. This prevents unauthorized users from changing the FDN list. A sensible use is z. B. Parents restricting the number of children's phones.