How can I preserve my voice while cold-calling?

I have a part time (once a week, 9 to 5) job cold calling. It’s from a home office and is very informal—my boss lets me take breaks when I need to. But towards the end of the day, my voice is SHOT—I can barely choke out the words.

Is there anything I can do to enable me to make calls throughout the day without dying? I cannot quit this job or ask for shorter hours, because I really need the money.

Thanks!