Quite often these companies use what is called a telecaster to phone you (at different times of the day) to determine whether you are at home. How often have you answered the phone only to hear a beeping followed by an immediate hang up?
Apparently the telecaster's software uses certain algorithms to differentiate the difference between a recorded message (voicemail/answerphone) and words used by people to answer the phone. However, there is a ruse you can use to try and protect yourself, I.e. when you receive a call like this - listen but don't speak, if you hear an feint beeping then press the hash key on your phone rapidly as fast as you can. This can confuse the software at the other end and effectively scramble the monitoring.
Another ruse, is that when you receive a call from telesales, get the name of the company at the beginning of the call and (when you have this), stop the caller immediately (talk over them if you have to - but be firm), tell them that you are registered with the Telephone Preference Service and they are in breach of same and that you intend to report them with a view to prosecution attributed to harassment. Believe me this works - they very rapidly lose interest - I very rarely get call now!
Lastly get yourself registered with the TPS (URL in the earlier thread) as soon as you can. It takes a little while to be effective, but is well worth the wait.
That's it and Si thee