"a quick mention that this number is on the Federal Do Not Call list sends them into a near panic state, scrambling to hang up"
Really? I've telemarketed before in my dark past. When people told me they were on the do not call list I would say 'I don't care' and would go into the pitch. Then they'd hang up on me. It was just fun when people thought they could thwart me by being smarmy or clever. I hated my job and all those who I had to deal with on the phone. So anybody who tried stuff like the 'do not call list' automatically wound up back in the caller registry. Your best bet is to just hang up the phone.