Software and other services are available to help block these calls, but they mostly do not work because the callers frequently change their numbers or even use false spoofed numbers. Also, customers must spend considerable time constantly adding unwanted numbers to their list.
Clearly we need a paradigm shift that I am confident could be accommodated by telephone service providers, if regulations simply permitted them to offer this service. This is the change needed:
Optionally allow customers to provide a list of valid numbers. All calls from numbers not listed should be sent directly to the customer’s voice mail without ever ringing the customers telephone. I am certain this can be done because it would be very similar to the largely available service of providing a list of numbers to block. Just allow the valid numbers and block all the others from getting through.
Likely very few robocalls or telemarketers will even leave a voicemail. Problem solved. Spoofing numbers should also be illegal though that would not matter much if this change was permitted.
Your support of regulations like this will be very much appreciated by many more than you could count, including Democrats, Republicans, Green Party people, and Independents.  Tell your congressman and phone company to offer this now..