Does the phone ring, you pick it up, noone is there and you hang up? Chances are you are on an automated call list for a company trying to sell you something. A computer calls numbers at random and then puts the call through to an operator if it gets through.
2 suggestions:
First, get your number logged with the telephone preference service which is free and will help for the future.
http://www.callpreventionregistry.co.uk/
http://www.tpsonline.org.uk/tps/
Second, just once, answer the call and talk to the operator. Politely request that your number is removed from their database. Ask the person�s name and the company�s number. Say if you are called again by this company you will make a complaint.