Called twice and did not leave a message.  Likely another scammer.  Number now blocked.

Received a call from this number 18 hours ago. Did not answer.

I screen and block, and never take calls I don't recognize.  Long ago I disconnected my voicemail, having tired of finding its memory filled up by sales messages and political robo calls.  But I understand from the comments of others that seldom is an actual human on the other end and never does anyone leave a message.  Can someone explain what is the point of all this pointless  pointless calling?  How can it be cost effective?