unknown number, who is this, call back no answer and cant leave a message.

the man on the phone is a dirty muthafuhka. hang up right away. very bad man.

Spam number with no one on the other end. When you call those numbers back, they all go to the same voicemail.

Those calls are coming from the same provider, VeriSign. NOTE: VeriSign said they are not the spammer and don't make outgoing calls, but the lines go back to them as the provider, like Verizon is a provider, not the telemarketer. VeriSign isn't a phone provider, they are internet security, but some telemarketer is using them so they can't be found.  Call VeriSign and report the numbers.  More people that do, the more they might look into it and stop the telemarketer who is doing it.  I gave seven different numbers to them, including this one.