Great answer! Yep, its a very OLD tactic that scumbag collection companies use. the debt is NO longer valid. they buy them for pennies, use volume/auto dialers to reach as many people as possible. NEVER answer the phone, and if you do, hang up once they identify themselves. IF debt is old you owe nothing!
Call blocker will help some, but these guys have hundreds of thousands of numbers to use, so they can overwhelm your call blocker, unless you refuse to pick up on calls you do not recognize. That will defeat them.