National Do Not Call List Registry
I signed up for this years ago, and it has been wonderful not being plagued by so many calls.
I saw the site mentioned in another thread, and thought it worthwhile for the site to have a thread and clickable link of its own.
I noticed too that you can file complaints at the same site, if you get calls from offenders and have been on the Do Not Call list for 31 days.
*Reminder: Even if your number is registered, companies with which you do business may continue to call you. So may charities, political organizations, and telephone surveyors.
You can register at the site, or check your registration, or the registration of others.