Are you sure she did not hit face time by accident? Could it be a butt dial and your over thinking it? Was there more than one attempt?
She had blocked my mobile number, I know this because a few months ago i tried ringing her and it went straight to the busy dial tone, then called withheld of a friends phone and it rang, she has an iPhone like myself, so to call me she would have had to:
Go to settings
find contacts
go to blocked list
click edit
delete my number from block list
then make call
So i know for certain this wasn't an accidental call