From the experts:
Calla is a lily name that is much more distinctive and delicate than Lily. Rarely heard today, it did appear in the popularity lists in the last decades of the nineteenth century.
Calla is the eponymous heroine of the Rebecca Wells novel The Crowning Glory of Calla Lily Ponder, and there's an English rock band called Her Name is Calla.
Actress Mare Winningham named her daughter Calla way back in 1987. Sound-alike Cala is an Arabic name meaning fortress.