In Germany, the "calling name" is underlined on the birth certificate. For example, up until a few years ago I had no idea that my mother's first name was Herta, which I had believed to be her middle name! She goes by her "middle name" Martina - and has been called that since the day of her birth!
Additional tidbit of information that I also only found out a few moths ago: the official lists of most popular German baby names are compiled of both first and middle names. No wonder Marie/Maria is always so high up there!