I get several a day on my cell phone, which is doubly illegal since it's on the do not call list, and as a cell phone, robo calls to it are illegal. But that doesn't stop anyone.
And blocking the number doesn't help much either. They call in with a called ID using a random, unused phone number. So the incoming caller number is constantly moving. I've tried calling the numbers back a number of times and they are always unused numbers.
The best I have coming up with is to do sort of the opposite of blocking spam numbers. Rather than blocking known offending numbers, I only answer known and recognized numbers. if I don't recognize the incoming number, I simply don't answer. Incoming calls are guilty until proven innocent. I figure if it matters, they will leave a message. I have yet to miss a legitimate call, which shows how bad the situation has gotten.