They're allowed to say if the information isn't available (an example given in this article about AQA (http://news.independent.co.uk/world/science_technology/article6520.ece) is "How tall is Mrs Thatcher?"). Also from the same article: "The service offers a money-back deal for anyone not satisfied with their answer. If the researchers can't answer a question, they always reply with some relevant and interesting information, or text back within the allocated time to let questioners know that they are still working on it."

I don't think I could cut that down to 160 chars, but I hope it answers your question. ;-)