If it is an outbound call centre call, I like to say:

"is that so?" a few times, then in a gravelly voice "what colour underwear are you wearing?".

Usually followed by a silence and them asking me what I said, so I ask the question again.

That is usually followed by beep beep beep beep.

Although with the "I'm from Windows Support" scam calls I usually go down a different path. As they normally say "your computer has reported an error" my reply is which one?".

Their script then tells them to say "your windows PC" to which I reply "which one?".

This can go back and forth a bit then I say "I have 37 PCs running in my server room, if you're so f'en clued up which f'en one is it?".

It all goes downhill from there...