Unlisted Number Still Accessible?`

Last year, I added the unlisted number feature to my Cox home phone service.  I still seem to be getting calls from people (real estate agents) who know my phone number and who I am.  Is it just that these people got my number before it was "unlisted" or is this feature not working?  Some of my calls are just robocalls that dial everyone in an area code, and I know those can't be prevented this way - but it's curious that a real estate agent has my number ("we just sold a home in your neighborhood").  Our number is also in the do not call registry.  I actually answered the last call, told them that since they were calling an unlisted number and I was in the do not call registry that I would never do business with them and asked them to stop.  

I'm just curious - can people look up your name and number from the "old" phone listing data even if you are currently not listed?

David