Keeping tabs on the times there's a live voice at your number. That way they can resell your number so other nuisance callers know when to take a chance. If possible, no kind of answer thwarts callers from gathering anything from your number. A number that always has an answer is far more valuable than one that only rings.

did not solicit this call and do not want to be called from them.

What I do talk to the Sales rep. That this is the 3rd call so you can come an install it. Give no info you talk to the installer payment. They been calling me 2.5 years been out twice. ...

RoboCall and the transition to a 'live-answer' is delayed, so it appears as no one is on the line when you answer their call.
Received a call from +1-346-309-0008 with a Texas ID.  Person identified themselves as 'Home Security', but when pressed for the name of the company, identified the company as 'Home Protection Technologies' and went back to his scripted monologue about a free home security offer for the homeowner, and asked if I was the homeowner.
When I asked where their monitoring station was located and if it was UL-Listed, he could not answer and I heard someone, in the background, feeding him responses.  Asked to be transferred to the person he was speaking with and someone came on the line stating he was a manager.  When I asked him where their monitoring station was located, he stated they had 8 locations, and when I inquired of the location for the one in Florida, he hung up on the call.