..because they will be selling your details on to over 1,000 companies & you'll never get rid of the nusance mail/emails/phone calls...
I had a cold call today from somebody wanting to fit me a "smart electric meter" for free, one that sends your usage direct to the supplier. The sent a letter a few weeks back so pretty sure its legit. And it's free. "yeah, sure, come round any time" I say, "let me just check the diary sir, ah, we don't have any engineers in your area for the foreseeable future so I can't book you in". SO WHY PHONE ME!
