Well, what options does one have to defend themselves against intrusive callers?

? The Do Not Call list (Federal or State)
Works, but is effective against ONLY "legitimate" telemarketers.  Political callers, surveys and some other callers are exempt.  Scammers ignore the DNC list.

? NoMoRobo
A system offered by some telephone providers, to help automatically block robocalls.  Google "nomorobo" to see if it works with your phone provider.

? Shouting, screaming, swearing at the caller
Nope.  Does not stop calls.  May make you hoarse, and will disturb nearby people.

? Call blocker app, device or service
Could be a good choice (even though the scammers can use fake phone numbers), and some apps are free!
Google "call blocker" for more information, or check your favorite online sales site for a list.
http://gg.gg/blockers
Your phone provider may also offer blocking services.  Check their web page for information.

If you cannot use any of the above defenses, then:
? Do not answer calls from unknown numbers ... let them go to voicemail or answering service
? Do NOT return calls from unknown numbers!  All that does is verify your phone number!  And, in some cases, those return calls may cost $20 or more to complete.
? If you have a "dumb" cell phone, assign the scammer's number to a group named "scammers" (or similar).  Then, assign a silent ringtone to that group.

How many times does one have to say "STOP CALLING THIS NUMBER!!!" before the Indian salesperson will stop calling and trying to sell us software and consulting services.

We're not buying your crap, dude!