If you want to know why you sometimes get only silence when you answer the phone, you need to understand how telemarketing "call centers" work when they use modern dialing machines to save time for their human employees.

For a complete explanation, see the Wikipedia article Predictive dialer, especially the paragraph Silent calls.

If an ordinary person is calling you, they will probably say "hello" less than a second after you say "hello." If there is a delay of even 2 or 3 seconds, it's probably a machine calling you.