The only way to prevent these calls is a call blocker, however. Scammers can either spoof 1 of 10 billion numbers...or to not use a number at all (Private, Unavailable). It's improbable you'd be able to defend your phone.
However, if you purchase a call blocker, you could configure it to only allow certain numbers to ring your phone and leave messages on your answering machine. It's an Allow List. You'll have to do some research. Review as much of your call history as possible to determine who calls you. If you know of a potential number, add it. If a known number calls you, add it. If you're expecting a call from an unknown number (contractor, family traveling, etc), temporarily disconnect your call blocker.
The following Cox Digital Telephone features may help you gain a better control over incoming calls:
Long Distance Alert
Selective Call Acceptance
Selective Call Forwarding
Selective Call Rejection
Anonymous Call Rejection
You may already subscribe to these features and simply need to activate them. Click HERE to learn more about the tools available to manage calls.
Make sure all of your phone numbers are registered on the National Do Not Call Registry at www.donotcall.gov. Spoofing technology allows scammers and telemarketers to mask their true identity, so unwanted calls may get through despite all your attempts to block them. Learn more at the FCC's Consumer Help Center.
Ugh...identification codes, feature codes, option codes, wait for announcement, follow instructions, enter more codes...how long does it take to block a number?
Where are the prices for these features? How many numbers can I block...20? After I reach my limit, how do I remove numbers? Do I have to pay to block numbers that don't even exists: 000-000-0000...123-456-7890...my number?
These features were designed 30 years ago to combat live telemarketers on the analog Plain-Old-Telephone-System. They do nothing today against computers, software, spoofing, war-dialing, fake area codes, fake exchanges, custom name fields...
The No not call list is a joke and does not work and who has time to report. We have lives. NoMorRobo solves this problem. Your option I am to put in 30 numbers. I have customers that call my home, that would never work. Just support NoMoRobo.
They have done all the work for you. Or lose customers who want this feature.
I actually had Cox turn off ALL in-bound calls - no charge for that. Everybody in the house uses their cell phones, and all I really want phone service for is emergencies outbound. (I have no "extra frills" on the phone line, so I can't even call long-distance.)
Now all callers get "The phone number you reached is not in service." Period. No charge for this service!
Also, keeping the phone plan is somehow actually CHEAPER than getting rid of it. (You lose the bundling discount, which is larger than the cost of the low-end phone service!) So if everyone has cell phones but you still want an emergency line for outbound local calls, ask Cox to turn off in-bound calls.