Nuisance calls, which can be anything from unwanted sales calls to outright fraud, are difficult to avoid entirely but can be minimized by using the Telephone Preference Service (TPS), which is always free. One current scam involves calling people, telling them that their TPS enrollment has expired (it does not expire!) and asking for a credit card number.
Telephones which are registered with the TPS should not receive unsolicited sales calls. You can register online at TPS Online. The website has lots of good information about sales calls, fraud, and scams. Go to the section entitled “Still Getting Unwanted Calls?” to find out more.