that means you also have their number right?
make a craigslist listing with content like "free concrete mixer" "free plushies" or something like that and their number as contact info, won't work on robocalls probably
the effect is twofold: first they will get calls (originating from real numbers) from cheapskates wanting their freebies. then their number may be picked up by a crawler and get cold calls from unrelated spam operations. this all in addition to figuring out what just happened
if you get it right you might force them to change phone number
while it's a certified chaotic move, this might be not entirely legal