Ignoring unknown numbers can lead to missing very important calls such as calls from a health system/hospital where there are many extensions that can be used. Buyer beware of this advice.Do not assume that you are getting calls to previous owners.
A technique that telemarketers are using now is to call a phone number and intentionally leaving an important message for a non-existing person, hoping that you will call back which services a few purposes... 1) it verifies that your number is a valid number, 2) you may inadvertently give out personal information (such as your name), and 3) it gets you on the phone...
"Hey John, I'm calling you about your job application. We are offering you the position but we need you to contact us immediately to verify a reference. If we do not hear back from you today, we may have to move on."
You call back to say it's from a wrong number. They will then start asking you for information and if your guard is down, you may have given them your name, etc.
We also do what Neil does.... any number that isn't registered or known is ignored.