Apparently, if theres nobody on the other end when you answer its an automated call thats checking to see if the number is active & at what time you answered. This is then used to contact you in the future when you are most likely to answer your phone.
Register with the TPS:
http://www.tpsonline.org.uk/tps/Quote from the web site: "The Telephone Preference Service (TPS) is the central opt out register on which you can record your preference not to receive unsolicited sales and marketing telephone calls to your home or mobile telephone numbers. It is a legal requirement that all organisations (including charities, voluntary organisations and political parties) do not make such calls to numbers registered on the TPS unless they have your consent to do so".
Anyway, if you don't know who a caller is, why answer your phone? As I said before, if its important, about work or a friend of a friend they will leave a message.
Since my original post I have still received a call a day from this number and am registered with TPS !!