It seems to be a market research company - cold callers.
http://www.dynamoo.com/diary/lbm-0608.htm
There is nobody on the end of the phone because computers are dialing literally thousands of phone numbers in the hope that one will be answered when a call handler is free.
They give a phone number for you to call back for one reason only - to get round the 'block anonymous callers' and 'anonymous caller rejection' many people have.
Take comfort in the fact that every time you dial that number it costs the company money.
Consider joining the Telephone Preference Service:
http://www.mpsonline.org.uk/tps/