An anti nuisance call policy that has served me well and I try to get my folks to adopt is that if there is the merest hint of a delay between my hello and the caller's response, I put the phone down immediately.
I don't think I've ever had an identifiable repeat call, from which I conclude it's both effective and has a low false positive rate.
I take this a step further and don't say anything for the first few seconds if I pick up a call from a number I don't recognize. I used to not pick up those calls at all, but now that spam callers are using local area code caller IDs more and more, that is getting more difficult.
I don't think I've ever had an identifiable repeat call, from which I conclude it's both effective and has a low false positive rate.