Why is this even a problem? Doesn't everybody have caller ID nowdays? When telemarketers, or whoever they are, call me, If I don't recognize the number, I don't answer.
That's what I do; NO-ONE who's not already in my phone with name and address and picture and whatnot gets through to me!
I quite often have to take calls from people behind switchboards, so they tend to appear as "Unknown". Bit annoying.
Now, that's an app I need to get, thanks Holdfast -I didn't even know this kind of software existed!
I just looked on my phone; the software I found is called Call Firewall.