Calla comes from the Greek kalós, meaning "beautiful" — and is the name of the calla lily, the elegant trumpet-shaped flower. The botanical Latin name is Zantedeschia; the common name Calla stuck from the Greek root.
Calla as a girl's name is rising — currently rare in the U.S. but distinctive. The character Calla in the novel The Lone Wolf brought it some visibility.
Calla reduces to seven — the number of quiet beauty.