Those numbers are the telemarketers from all fields like banking, finance, fitness, beauties... There is a database built up especially for this kind of calls from these telemarketers (junk calls).

I just tried some hundred numbers import and it works fine. Of course I haven't tried all 9k numbers yet so don't know if it will work really slow

I may have new customers calling in from time to time with numbers shown as unknown or not in my contact list. Whitelist seems not good option for me in this case. Therefore I want to make a good use of this nice callblocker with the database to filter all these junk calls.