My reaction to all of these callers, and the same to PPI scams and others, is to waste as much of their time as possible. Just hanging up is exactly what they want, so they can move on to the next sucker. Instead, ask questions... ask them to 'say that again', ask them to 'talk more slowly', tell them to hang on while you answer the door. Tell them you don't speak English and ask them to talk in various random languages, then explain you've only just started learning, and then try a new one.
You'd be surprised how long you can keep them occupied, and wasting their time
Will destroy their business model.
The real satisfaction, which I have achieved a number of times from PPI 'helplines', is to be sworn at as the caller twigs they've been suckered. Yeah, result!
PS: re PPI calls, you do have to press '5'. Don't worry, contrary to urbany myth it won't cost you a penny on a BT landline, and you really can have an awful lot of fun.