I am getting network error when try to set call forwarding when unreachable . can any one help why I am getting this error

Block Calls from a Contact:

1. Tap “Apps” on the home screen
2. Open “Contacts”
3. Tap the name of the contact you want to block
4. Tap “Edit contact” (the icon shaped like a pencil)
5. Tap “Menu” and enable “All calls to voicemail”

NOTE: To unblock a contact, follow the steps above until the last one, then tap “Menu” and clear the check box associated with “All calls to voicemail”

Forward All Calls (Unconditional):

1. Tap “Phone” on the home screen
2. Tap “Menu” > “Settings”
3. Tap “Call settings” > “Call forwarding”
4. Tap “Always forward”
5. Enter the number to forward calls to and tap “Enable”

Forward Calls (Conditional Forwarding):

1. Tap “Menu” and go to “Settings”
2. Tap “Call Settings” > “Call forwarding”
3. Select to forward calls “When busy”, “When unanswered”, or “When unreachable”
4. Enter or update the phone number to which you want the calls to be forwarded
5. Tap “Update”

Cancel Call Forwarding:

1. Access the “Settings” menu
2. Tap “Call settings”
3. Scroll to and tap “Call forwarding”
4. Select “Always forward”, “When busy”, “When unanswered” or “When unreachable”
5. Tap “Disable”