Because when you answer your number is listed, often by a computer, as an answered live number. They can then sell it in a list as a confirmed current live number. Once upon a time you would get a network or fax login squeal seeking networks to connect too. Now it can be many possibilities.
Unknown number, left no message, I did not respond. Thanks for leaving comments!

the caller is already blocked and still they try and call.

Caller: unknown

I called the number back and recorded male voice says they are fully compliant with the laws pertaining to the don't call register. WHAT A LOT OF ----------.

Rings repeatedly and says nothing if you answer. Yes, easy to block but the numbers keep changing

7 calls in one day!! never leaves msgs...

Recorded message saying this is a survey about politics.
Hung up at that point!

"Hello, this is a short survey ..". I hung up, as I am sick of these calls. They should be banned if phone number is on the do not call register. Alternatively, the do not call register should be expanded so that you can optionally ban all the serial pests, such as these.

AGAIN - they call, I answer, they hang up

SPAM. hung up when answered

Automatic answer about a short government survey

Caller: (02) 8015 6802

The phone doesn't even ring but I get a message from message bank that I have a missed call from this number?!?

Answered with my business name and they hang up! Could just say "sorry, wrong number" or ask if I'd like to be removed from their call list.

3 calls in 20 minutes ... not answered by me as I am at work and all calls go to message bank during work hours unless I recognise number. Each time rang out and no message left ... going off posts on here they are obviously spammers and have been blocked on my phone forever

recorded message claiming to be doing a survey

SPAM: calls and hangs up upon answering. So WHY are you calling at all?!

Caller: (02) 8015 6802‬

Missed the call and got a voice message. It was Michael Daley from the Labor Party inviting me to a live town hall recording of Labors plan moving forward. It was a recorded message

Caller: Federal Election Polster.

Did the survey.

Got a call, didn't answer, they didn't leave a voicemail.
Lucky I checked here and lucky I didn't answer.
BLOCKING!

Caller: Batman

You’d be Bat’s to answer 🦇

Called and left no message. I blocked number.