Okay people, here's what you do. when you get a call from a number you do not know, DON'T ANSWER IT! If you answer it, your phone number gets recorded as a legit number and you'll start getting calls from a lot more numbers you don't know. That's why these call robots exist, they sell phone numbers that have been verified to other scammers in bulk.
What you do when you get a call from an unknown number is check this site. If a bunch of people have said "I answered and they hung up right away" or "scam" or whatever, then just go to your mobile carrier's website and block the number. It's that simple.
I don't know how other carriers work, but with Sprint you'd log in and go to preferences and then click block voice. Select your phone, type in the number, hit save at the bottom and boom. Done. I currently have 28 numbers on my block list. Every one of them I checked on this site first and every one of them is a scam or a callbot.
The reason these callbots and scams exist is because it is not financially beneficial for phone companies to stop them. This will never end, so learn how to deal with it intelligently.