The call is unlikely to be a fraud unless you notice certain warning signs, such as being asked for money early in the conversation. Remember that you do not have to buy there and then. Be suspicious of telemarketers who sound like they're trying to force you to do so. You can always tell them to mail you information and then decide later. Assuming you have a human on the line, please remember that no matter how angry you may be about being dragged out of the shower, the human talking at you is not the real cause.
Many people just hang up without saying a word as soon as they recognize that it's a telemarketer.
This won't get rid of the problem because it leaves the company with the legal right to call you again. It's possible they will take that as an invitation to call back another day to see if you've changed your mind. If you don't want them to call again, you must say so clearly. In the United States, the organization is then required by law not to telemarket you on that number for the next 10 years. If you have more than one phone number, you may want to tell them the other numbers as well.