NEVER answer, even worse respond to, a number you do not recognize. Same as using the Unsubscribe for E-Mail, you are letting someone know that your phone number is a working number and they sell it to advertisers.
I created ZZZ as a contact and programmed it with a silent ringtone. Whenever I get a questionable call, I do not answer it and if I figure it to be a junk call, I save the phone number into ZZZ. This way, if they call again, I see ZZZ on my display and know not to answer it.
This was started over two years ago and I now have over 145 entries in ZZZ. I rarely ever see ZZZ on my phone. This proves that blocking these calls is worthless.
Just don't answer numbers you do not recognize. If it is important, they can leave a message.