I occasionally get calls that have no one on the other end. I have concluded that those are robo calls designed to determine whether particular phone numbers are answered by ‘real’ people or not. If so, the number gets sold to advertisers/scammers for premium prices.
Most times, I do not answer any call unless I know who is calling. I only have Tracfone and use their CallDetector app. The app works pretty good to signal who might be calling.
I wondered what the point was of those types of calls. I have a landline that cant block calls. So I bought a phone that requires you to punch 1 to get through. Solved the problem.