@Richard good heads-up.
My favourite response to cold callers (and I have responded to a few unknown numbers when I was expecting a legit call) is to be completely polite, cheery, helpful and then ask
who they wanted to speak to as 'we're very busy in the office today' (I don't work in an office).
It seems to work well. "Well if you can find out who you needed to... please call back... thank you for calling
"
Oh, and then I block the number.