, plug in between your phone and the wall jack. Capacity: Many come pre-loaded with thousands of known scam numbers and allow you to add thousands more yourself. One-Touch Blocking: Most of these devices feature a large "Block" button; press it during an unwanted call to hang up and permanently ban that number. 4. Register for the Do Not Call Registry While this won't stop illegal scammers, it will significantly reduce legitimate telemarketing calls. Registration: Visit the National Do Not Call Registry or call