I am not much experienced with callas (only two years)but my methods work.
I leave them in the garden until the first light frost and then take them to the basement for a week or two (first drying not too fast) and then I take them (in the pots)to my flat (temperature about 20 centigrades). I keep them dry until the first shoots appear and from that moment I start watering. After last late frosts I take them to the garden.
Good luck !

Call me brave, but I'm trying to leave mine in the ground this year with a very heavy mulch (they have total southern exposure). I'd kept mine in a pot for 3 years and brought in over winter and had 1 small bloom this year. I figure if they make it great! if not, I'll start over next year. They just take up too much valuable real estate during the winter for one small measly bloom.

Cheryl