On my phone I am able to send contacts straight to voicemail.  I basically make a contact name called (Do Not Answer) and put that number into that contact.  Then I can set the contact on my phone to go straight to voicemail.  When ever I receive any type of calls like these I just add it to that contact and never had to worry about them again because none of these people leave messages.  If it was a mistake and they do leave a message you can then remove that number from the contact.  Hope this helps.