>> How can we stop this?  <<

Have you considered obtaining a call blocking device, app, or phone?  Google (or your favorite search engine) IS your friend!
Realize that the Do Not Call list is NOT a call blocker!

Here are some tried and true methods for reducing or eliminating repeated spam / scam phone calls:  Click on the Info link, below, to see them

InfoWant to stop repeated telescam phone calls?
Try these alternatives:

1.  Do not answer the phone if you see an unknown number on your Caller ID ... let it go to answering machine or voicemail.  Many telescammers hang up after three or four rings.

2.  If you have a smartphone, get one of the inexpensive or free phone blocker apps at your online app store, and learn how to use it.

3.  If your cell is not "smart", read the manual to learn how to set up a "group".  Assign the scammer's phone number to that group.  Also assign a "silent ring" to that group.  You will not hear your phone ring if that number calls you again.

3a.  Alternately, assign a "silent ringtone" as the DEFAULT ringtone.  Then, assign your favorite ringtones to friends, family, contacts, businesses, etc.  That way, you will not hear your phone ring, even if the scammer changes numbers!  (Disadvantage: if you forget to whitelist somebody, you won't hear their calls, either)

4.  For cell phones or landlines: Contact your phone service provider.  They may have a facility or method to allow you to block numbers.

5.  If you buy things online, you might want to google "phone blocker" to see what devices or apps Amazon or other sites have to offer for landlines or smartphones.

6.  DO NOT call unknown numbers back without checking who called (800notes is your friend)! All that does is verify your number is "live".  Instead, read this:
http://www.consumer.ftc.gov/blog/one-ring-cel ... ing-your-wallet

7.  Consider picking up the phone, then immediately hanging up.  A "real" caller will usually  call you again, within seconds.  A robocaller will not.
If you want to report potential fraudulent or scam calls, here are two pages of links to (mostly government) sites in Canada and the USA were you can report those scams, or learn more about them.

1.  Look elsewhere on this page (probably to your right) for the Article, "Where to Report Phone Crime"

2.  For a more frequently updated site, look here:
http://phonehelp.2truth.com/complaint_sites.html

For info on FTC and communication initiatives to stop robocalls and other illegal calls, click "FTC", below.

FTCThe FTC and top communications companies met, to determine what they can do to stop robocalls and other illegal calls.  Here is a link to a frequently updated page, showing the history and progress of their initiatives, as well as recent Congressional bills and industry implementations.

http://www.phonehelp.2truth.com/initiatives.html

(I strongly suggest you read the history and progress of
H.R.2669 - Anti-Spoofing Act of 2015
and
S.2558 - Spoofing Prevention Act of 2016
to learn how [slowly] bills progress through our Congressional system.)
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Please reply to this post, on 800notes.com, to let us know you read it, or have taken suitable actions to protect yourself.