Bob, that's very interesting. I was actually looking into doing something similar as a bit of a side project. Obviously, I'm not as far along with it as you are, but for those who are interested, here are some of the issues I've run into so far.
The best voice recognition software like Google, Alexa, Cortana etc generally wants to tie you into a web interface of some kind, which is a little questionable if you're operating in a bit of a "grey" area from a legal viewpoint and charges per recognition. Obviously this is not optimal for making massive amounts of nuisance calls to scammers.
There are plenty of less advanced voice APIs out there but the issue is getting them to work well enough without significant amounts of training it to a particular voice, which is something I doubt the scammers would be willing to cooperate with. The way to do it, I think, is to narrow down the list of words it has to recognise dramatically. The vocabulary it has to be able to recognise to interact at least somewhat with scammers is pretty limited and I think with a bit of "Lenny" style clever scripting I can see no reason why it should not work.
As Bob hints at, other issues are computational load and integration with SIP/Asterisk. Issues with those would probably limit the potential for mass calls but it would be an interesting project to see how long an AI bot could drag out a call with some scammers. Some tricks I can think of to drag out some calls further might include a Kitboga style "hang on, I've got another call" with either a real or fake conference call to another scammer. Another possible avenue to explore might be to have the bot integrate with a virtual machine so that it could, say, open a fake bank page or even fake redeem some google play cards or something like that. The biggest challenge I can think of there would be to have the bot be able to read off a Teamviewer or Anydesk ID so that the scammer can connect but I can think of a couple of possibilities.
As to bots, let's not forget the potential of Soundboards to annoy. I've just set up a bot using a script with some sounds from Count von Count from Sesame Street (http://www.realmofdarkness.net/sb/ss-count/). The script goes something like this:
I am the Count
Do you know why they call me the Count?
Because I love to count
I am counting
Not now children, we will play later
This can run in an Asterisk dial plan script somewhat similar to how Lenny works.
I can also see potential in Chewbacca, http://www.realmofdarkness.net/sb/sw-chewbacca/ and some kind of fart bot such as https://www.soundboardcity.com/fart-machine/.