As soon as I answered there was a delay of a few seconds and then they hung up. Why do these scammers call you and then hang up? We are getting so many of these on both our mobile and our landline

I answered and got the "This call may be recorded for training etc"
I hung up and blocked the call

Missed call at 6:21pm. No message left. I have no idea who or what called but I will not be calling back. If it was important, they could either leave a message or retry later. My guess it is a tele-marketer or a survey, neither of which I will respond to.