The OFT said it and trading standards offices have received thousands of complaints from businesses about the selling techniques of telesales workers. Many, it warned, make bogus claims about links to the emergency services or charitable organisations.
Sir John Vickers, OFT chairman, said: "Small businesses are particularly vulnerable to these types of unfair business practice. We advise all businesses to be wary: don't be pressured or tricked into agreeing to advertising you don't want or into paying for something you haven't requested.
The OFT advised managers not to not take unsolicited calls when they are too busy to give their full attention or agree to something just to get rid of a persistent caller.