IPhone :: What Happens On A Facebook Call Not Answered

I have an iphone 4and my niece has an iphone 4 . We were discuusing facetime and she has never used it . I have used it only twice . I tried to explain to her how it worked . and i said would try and facetime her sometime when she was home on her wifi network . i checked her phone and facetime was on I face timed her mobile # and it just rang so i assumed she was not there i ended my call to her

Last evening around 10 pm my iphone rang (I never get calls at that time ) when I answered it was a facetime call from her . She was in bed and wanted to know what i wanted . i said she called me . She must have been using her i phone and somehow called me .She was confused as to how she did it

Info:
iPhone 4, iOS 5.1