You are right, it is a Calla Lily, probably Zantedeschia rehmannii, or rehmanii. For them to survive the winter there they must be planted at just the right depth in just the right spot. Take care of that treasure!
susan

Thanks for the reply. I plan to put plenty of mulch down this fall. They're so beautiful, I may even plant more callas in the spring and see if they can make it through the winter.

Aren't they gorgeous? There's a lady in Oklahoma who grows and overwinters her in zone 6b. They're beautiful, too. I am in zone 7a, and planted mine for the first time this year, so we'll see if they overwinter.

Susan