What kind of calla is it? The one with large heart shaped leaves are hardy to zone 7. And ones with lanced shaped leaves hardy to zone 8. So you would be able to plant directly in the ground. Unless you just prefer a pot.
Repot it and add some coarse builders sand or very fine gravel or pebbles to good potting soil. Or perlite if you have it. Keep moist. But if it dries out a little it will not do much harm.

Here is a link that might be useful: calla planting

Thanks for the advice. It makes sense that sand would make the soil drain faster. I used containers because I'm gardening on a patio. I will try the calla planting link - thank you!