For iPhones
If it's a voice call you want to block, make sure you have that number in your Contacts lists. Then go to the Phone app, select the contact you want to block, tap Block This Caller, then Block Contact. To stop them from reaching you on FaceTime, go to that app, enter their name or number, and follow the same steps. For texts, go into Messages, select the person you want to block > Details > the "Info" button on the top right > Block This Caller > Block Contact.

For Android
Go to the Phone app and select Call Settings > Call Rejection > Auto Reject List. Type in the number or search for it, select it, and you're done.
For Windows

Go to Settings and then Call+SMS Filter, accept the Terms of Use and Privacy Policy if you have not already, then switch Block Calls to On. Now go to the Phone app, hold down the number you want to block, and hit Block Number, then OK.
Source http://www.timigate.com/2016/02/how-to-block-numbers-on-iphone-android.html#more