In practice, do you find this happening with legit calls where they hang up and call back immediately? I don’t experience it myself***, but I’ve seen reports where spammers will send two calls to a number at the same time so that while you are hanging up on the first one, they are leaving a message in VM with the second.
I’ve had a robocall filter for my main system for 3 years this month. It’s as simple as it gets, a prompt to enter ANY single dtmf tone and you’re in. I get a few live unsolicited callers a year and almost zero robots. I don’t think you need such an elaborate plan with adding numbers.
Robot challenges work well with the AllowList module. Once a CID is known to be good, it can bypass your challenge in the future.
*** I’ve since looked back thru calls that have been intercepted by the spam challenge and note that there are a fair number of instances where two or more inbound calls came in from the same CID in a very short time. Something like 5% of spam calls to me have this pattern.