Way back 2004, the Federal Trade Commision set up the Do Not Call Registry to protect phone numbers owner from telemarketers. If you are on the list telemarketers are prohibited from making telephone solicitations to your number as long as it remains on the list. Telemarketer will have to ask for your permission in order to call you.
To register online, visit the site https://donotcall.gov/ click the 'Register Your Phone' button and follow the instruction from there. You will be asked for an email to as they will be sending a verification link to complete the registration process.
You may also register via a phone call, just call 1-888-382-1222 or 1-866-290-4236. Make sure you are calling from the phone number that you wish to register.
Remember that the do not call registry only prevent telemarketing calls or sales calls. You may still received calls from unwanted callers, political calls, debt collection calls, survey calls etc. You may also still get calls from companies you have done business with. To stop these calls, you may write them a request to remove you from their list, they are required by law to honor your request.