Missed call from Private no.

I recently got missed calls from something called private no. but I was unable to retrieve even the number from my phone. It just showed 3 missed calls even in the log. When I browsed through the net I got to come to know that these are the calls from marketing companies probably situated outside our country. So we may face the problem of calls in odd hours. Just ignore the calls for 2 or 3 days and they might not disturb further.