|A pink Calla Lily flower
Calla has the dual advantage of a old-fashioned yet modern feel. It is easy to pronounce and feels warm and friendly, girly and tomboyish all at once. Calla, far from being a modern invented name, is a Greek name meaning beautiful. It would be striking with other nature names such as Sage and Bay. Calla is also perfectly poised to be a nickname for longer Greek names that derive from Calla - such as Callista (meaning most beautiful) and Calliope (meaning beautiful voice). Which also opens it up as an option for those that love more fanciful/mythological/Greek names that have more accessible nicknames.
A couple of objections I've seen are that it could be caught up in the Kayla/Kaylee trend, that it was the name of a Gummi Bear on the 80's cartoon, or that it is slightly reminiscent of "call girl". However the response to this name seems to be overwhelmingly positive, with most lovers of the name suggesting it'd be best paired with a shorter and/or more traditionally feminine middle name.
Whatever way you prefer to use it, Calla is a versatile name that is beautiful by name and beautiful by nature. I'd love to see used more often!