Q: Facetime to the US from UK

I'm trying to Facetime a friend in the US.  We both regularly use Facetime in our own countries but have never tried to each other.  We are using our Apple email addresses but keep getting messages to say that the other person is not available for Facetime.  I regularly FT my Dad on his ipod using his email address so what am I doing wrong here?

Apple geniuses (sp???) please help

Clare