Against the law to do the non-disconnecting calls:
While the article doesn't specifically address hanging up and they are still there, it does talk about the regulations. Not only that, but I remember (Can't find the source at the moment), that there was a law put into place that basically mandates disconnection if hung up on. This was brought to light by the need to dial an emergency number if need be and the caller wasn't allowing that to go through, then the caller would be liable. Since I can't find the source - probably something I heard, but do think some regulation is there for that purpose (would predate everyone having a cell phone on hand).
I used to have fun with robocalls - usually press the number or wait for the other person to answer (Depending on the robocall message), and play with them. Once gave a home refinance caller bogus information on some commercial property, over valued it and gave a phony name. Spent well over 30 minutes of his time feeding him the information including phony name and address. (he called my place of work). At the end of the call, he said he had to do a credit check for some refinancing figures, so after being asked for my social security number, I fed him the date in a manner that wouldn't be easily recognizable (something like 011 42 01333 which is today's date with an extra digit or two in there). Funny part was he didn't catch it and we hung up. About 10 minutes or so later, another coworker got a call looking for a man named "Rusty Boltz" (Yes I used that name), and was promptly told nobody here by that name, he rebutted saying he just spoke with them at the same number. We all knew what was going on in the office, but he really was upset that he was taken for a ride. We all got a laugh out of it and never heard from that caller or group again. Sometimes it is fun to play with the trolls.