There's nothing more annoying than being harassed by calls or messages from private (hidden) numbers. Luckily, most of Android's higher end smartphone models have a built-in feature to block incoming calls and messages from hidden numbers. The steps may vary slightly from one manufacturer to another, but in general, you should be able to access this setting from the Phone app. Here's the procedure for the Samsung Galaxy S6.
Block Calls From Unknown Numbers on Android
Open the Phone app, and tap More > Call Settings > Call Rejection: