Iphone 5 wont ring when getting calls from UNKNOWN

Hi All, I got my iphone 5 with iOS6 on Friday and have a strange problem. Not sure if any of you have had this. When i get calls from UNKNOWN number, the phone wont ring. It wont vibrate either. Calls from valid number rings though. I am not sure if something is faulty with the phone.

I get calls from Voip machines from my customers based out of other countries and miss most of their calls because my phone wont ring. Can you please advise what to do about this? Do I need to change some settings in my phone or is it a faulty phone?

Thanks, Prem