Remember, they called you uninvited – there is no need to be polite. These people can be the bane of your life with repeated calls.
Refer them to non-existent legislation that they may be breaking. Anything that doesn’t follow their script is disconcerting to a cold caller and an authoritative voice warning them of law breaking should stop them immediately.
My number is registered with the telephone preference service. By calling me you are breaking section 117 of the Telecommunications Act 2012. You are personally liable to a fine of up to £5,000 for a first offence or imprisonment for further offences.
I suggest you get your bags packed.
This is not true of course – it is just intended to stop the cold caller from pestering you.