My guess is it's the spammers calling through without caller id and phones don't know what to do.
In parting, reboot the phone and don't take this as a bug only for iPhones yet.
I have been using iphone for many years,yesterday I have faced huge problem with my iPhone,my iphone was ringing and vibrating once i got the call from my friends but my iphone doesn't show any number and strange thing was no button working. I was really in shocked mood. Is there any other option to get rid of this problem,i'm far away from service center. Any suggestion really be appreciated.