I too like to keep them on the phone, while they are entertaining me they are not scamming some unsuspecting victim, some of them are clueless and I get the impression they just follow a script, so you can baffle them with bullshit, I engaged the HMRC guy who's first question was, "what is your name" I don't think they will cancel my National Insurance number and prosecute me, if they don't know my name, after a while he gave some abusive comment and hung up. I will always waste their time, they don't have a problem wasting mine.