They likely just use an auto dialer with a predictive dialer that redials at preset times and then when blocked the program dials again with a spoofed #. My Panasonic phone with block is great but even when blocked it does ring "once " which I still find very irritating. I reckon that as long as there is that one gullable mark out of a thousand that gives them the card to max out these crooks will keep up the scam.
Well, shucks, I fantasize about the scamming business falling off and ending, but in reality it's just a dream. I maintain a log of unwanted calls. Some numbers call only once, and I don't block those. Unfortunately, after years of receiving far more scam and spam calls than real ones, I almost never answer a call. I call back those whom I know and log and/or block the remainder. That's pretty sad. Btw, your cat is nice too.