As fascinating these end-user solutions are, the best solutions are those which address the sources and enablers of these robocalls - the phone companies.
I recall watching on C-Span in the last few years some hearings about these robocallers and phone scammers. The phone companies have fairly easy ways to determine if there are many, many calls frequently originating from a single source (and a source not part of a legit exception, such as informational robocalls from local municipalities such as villages, counties, school districts; or medically based from doctor offices and pharmacies), making it evident that they are up to no good. The phone companies can and should shut them down.
Call number spoofing over the years has really juiced up the spamming because it can evade call-blocking programs. The phone companies, again, can stop this by preventing the caller from assigning a number to its call which differs from the actual number they are using. Yes, there are legit (and narrow) exceptions to this such as doctor's office which has multiple lines, and they want to have their main number appearing on the recipient's caller-ID; or from a battered women's shelter trying to conceal the presence of an estranged spouse. As long as the scammers can't obtain one of these exceptions, the number-spoofing can be prevented.
IMHO it's the source which should be used to put these spammers and scammers out of business.