If you receive a lot of spam calls from a specific area code and prefix, you have the ability to block that.
We call these calls the neighbor scam.
You can block these types of calls. In the app tap on the Block List (circle with the line through it along the bottom.) Then tap on the "+" and select "Numbers that begin with." You can then input any area code or prefix you want. You can also block by country code this way.
To whitelist your contacts you will need to update your settings. Go to the settings tab in the bottom right and then tap on call settings. Make sure and set "Calls from saved contacts" to do nothing.