Fixed Dialling Number (FDN) is a feature of a phone's SIM card that can restrict outgoing calls only to a special list of numbers, or to numbers that match a certain template.
A business can set up and use FDN as a security measure to prevent employees from dialling numbers outside the company from their corporate-paid account.
Note: Incoming and emergency calls are not affected by this option, so 911 and 112 are always reachable
To Enable
1. Go to the Dialer. Tap the Menu Icon in the top right corner > Settings > select Calling Accounts.
2. Tap the image of the SIM card you wish to configure
3. Select Fixed Dialling Numbers (FDN).
4. In this screen you can enable/Disable FDN, you will need to know the PIN2 code. You can also manage the list of FDN from this page.