Our number is registered with the telephone preference service- the equivalent of a "don't call" list.
I only have to interrupt the call centre operator by mentioning this fact and they apologise and ring off at once.
Since all calls are recorded, that means that they have just cost their company a fifty pound fine.
It has put a complete stop to the calls.
The phrase "We're moving away in a few months" gets rid of door to door Double Glazers and Energy switchers too.
Cheers
Dave