Very simple. They are scammers. Legitimate telemarketers follow the rules and regulations of the do not call registry. The people calling you are not legitimate, they are scammers using spoofed numbers and are often out of the country and out of our jurisdiction. Your best bet would be to invest in a call blocker if you are continually receiving unwanted calls.
Got a call again at work. Silence then a boop. "Hello can you hear me?". Again I remained silent. After a brief pause, the voice said my online registration had been selected to receive a free cash card. Would I like Walmart, Target, whatever blah blah blah." I remained silent and it went to dead air then eventually hung up.