This would not even go down as telemarketers. These are flat out criminals trying to rip people off. Telemarketers are actually legally trying to sell you goods or services.
Here's the thing folks, any telemarketer that is calling in violation of the Do Not Call registry doesn't care about any laws, as they are not legitimate telemarketers. Many of these robocalls originate from outside of the US using VOIP technology (Hence the weird Caller ID info at times.), which makes it tougher to go after. I would venture to guess that many of these mysterious calls in which you pick up and hear nothing are these operations trying to find numbers within an area code that are not in use so they can spoof the number. The DNC registry is not some magic bullet that stops all telemarketers from calling you. The ones that do are more than likely shady operations that try to obtain personal information that they can sell to identity thieves.