Your account by default is set to a Black-list, to change the setting to use a White-list, go to Callflow from the menu, select the tools icon and check the Block icon.
A white-list works where you add only the numbers that can access (call) your phone number.
A Black-list works where you add numbers you want to block from (calling) accessing your phone number.
A white-list is more secure because you are only allowing a list of numbers to call your phone number, if the caller's id number is not in your white-list the caller will be blocked.
If you decide to use a Black-list of numbers, the list of numbers wont be able to access (call) your phone number, any other number or caller id number not in your Black-list can call your number.
If you want anyone to call your phone number and only block spam callers, use a Black-list.
If you only want a small list of numbers to call your phone number, use a White-list.