The best way to store callas is to let them get just slightly blackened by the first light frost. Then, remove them from their pots, wash off the bulbs, and clip away the foliage. Then, bury them in a cardboard box filled with vermiculite, perlite or sphagnum moss. Place the box in a cool, dark location until spring. Do not let them freeze in storage. In mid-winter you might want to check them and if they are shriveling sprinkle them lightly with water and restore in the vermiculite.