I block every unknown number that is not in my contact list. With Android, you can do this by turning "Do Not Disturb" *ON* and setting appropriate parameters

I block every unknown number that is not in my contact list. With Android, you can do this by turning "Do Not Disturb" *ON* and setting appropriate parameters to allow known callers through as normal. Unknown numbers still show up in my call log so I can look them up online at my leisure to see who they are, where they're from and call them back ... or not. No paid service required in the USA.