Since it began in 2003, the National Do Not Call list has grown to include 229 million phone numbers.  Registering for the Do Not Call list makes it illegal for telemarketers to contact you by phone.  The Do Not Call list does not apply to charities so you still may be contacted by charities even if you have registered for the Do Not Call list. However, when you receive a call from someone purporting to be representing a charity, you can never be sure who is really calling so you should never give your credit card number to someone who calls you allegedly from a charity. If you are interested in a particular charity, contact the charity directly to make your contribution.
If you are registered for the Do Not Call list and you do receive a call from a telemarketer, you can be confident that the call is a scam because no legitimate telemarketer would call you if you are enrolled in the Do Not Call list. It is also important to note that while telemarketing is not in and of itself illegal, telemarketing through robocalls is always illegal.   Registering for the Do Not Call list will not stop robocalls. Illegal automated robocalls continue to be a major source of complaint for many people with the most common subjects of illegal robocalls being debt reduction, vacation and timeshares and warranty plans.
The FTC is reporting that people are now receiving emails informing them that their registration in the Do Not Call list is expiring and that they have to register their phone numbers again. They are then directed to a phony website that gathers information that can be used for purposes of identity theft. The important thing to know is that you never have to reregister your phone number with the Do Not Call list. Once you have registered a number it is permanently on the Do Not Call list until the number is disconnected or if you ask for your number to be removed.
TIPS
Registering for the do not call list is easy and free.  Merely go to http://www.donotcall.gov to register your phone number.
TIPS
There are a number of options for preventing robocalls including a number of apps that for free or a small fee will prevent robocalls. Here is a link to those for Android phones.
https://www.ctia.org/consumer-tips/robocalls/android-robocall-blocking
Here is a link to apps to prevent robocalls for iPhones.
https://www.ctia.org/consumer-tips/robocalls/ios-robocall-blocking
For landlines, Nomorobo offers its robocall blocking for free.  Here is a link to Nomorobo. I have used Nomorobo for years and find it to be tremendously useful
https://www.nomorobo.com/
Finally, you can just choose to ignore any calls that come from numbers you do not recognize.  If they are legitimate calls, they will leave a message and you can call them back.