I get these too, ring BT and get em to block private calls. [withheld numbers]

The best one i fucked around was double bloody glazing, they rang me, and asked how many windows i had... so i said hang on, and came back 15 minutes later [they were still on the phone... fuck they must be desparate] and said 4.

Most of the time as soon as they say Hello, im calling from so and so i tell them to fuck off, and if they carry on i threaten them with legal action, and that stops the calls for about a week, then it starts again.

Try and see if you can get yourself ex-directory. Then hopefully they wont get your number again and again.