Here's a quick and free way to send these calls straight to voicemail so you don't even have to hear them ring your phone. This works for Android phones, I don't know about iPhone but I'm sure there is a similar feature.
I created a contact and named it "blocked". You can name this contact whatever you please. (Telemarketer, Scammer, Low life, etc.) While creating the contact, select your menu, and check off "All calls to voicemail". Now, any number associated with this contact will not even ring on your phone and will be forwarded directly to your voicemail. In my experience, NONE of them have left a voicemail so these callers have completely disappeared from my life :)
Now - this is important. Any time a new call comes in from a suspect number, long press and "add to existing contact". Add the new number to "blocked"
My phone used to ring from these scammers about 4 times per day. After doing this, I've reduced the number of calls that get through to me to once per week at maximum.
Here is my list of numbers that I have added to my "blocked" contact so far. Feel free to start off by adding these numbers to yours for immediate relief from unwanted callers.