There are many people here in TN, zone 6 that leave their Zantedeschia in the ground successfully over winter. I try to dig mine (old habits die hard, I'm a long time zone 5-er) but have missed a few the last two winters, and they're growing just fine.
However, bulbs in pots (dry soil) inside a pole barn (unheated) were not as lucky. 99 out of 100 succumbed.