Simple. First, you have to realize that these scammers are all using some kind of VOIP system -- the actual call costs them nothing. They can cut a call short and try again later. And having a stored robocall message costs nothing either (in terms of paying a human to sit there and talk).
They are using what is called a "predictive dialer." They have already predicted that if they have, say, 10 outgoing phone lines (VOIP - connected computers actually), then maybe 2 of those 5 will be no-answers, 2 will be answering machines, 3 will be angry hang-ups...leaving four that will reach a potential victim. Let's say they have four humans standing by to scam you once you "press 1" or whatever. But now their predictions are off -- after they started ringing your phone, somebody picked up that they didn't count on, tying up one of the humans that the system thought would be free to handle your call. So it disconnects your call since there's nobody to scam you. This time. They will be back.
This also explains why some telemarketer calls have dead air when you pick up.
This doesn't matter. Log the (alleged) phone number and caller ID, time and date, anything else. The one call -- even if abandoned -- counts toward the two calls you need to file a lawsuit in small claims under the Telephone Consumer Protection Act and 47 CFR 64.1200, 47 CFR 64.1604, and 47 USC 227.
Another scam call from losers who leave an ear-splitting noise across the open phone line when they hang up just after my machine picks up. I could hear the screech at the other end of the house from the machine.
I keep getting their calls. I am on state and federal do not call lists. Neither works. Any retaliation anyone can take is justified. These beings should not be allowed to convert oxygen to carbon dioxide