Best prepaid rates to USA?

I'm traveling to Australia next week. I'll be there until the end of the month. I'd like to get a prepaid sim to call the USA.
Prepaid rates in Oz are confusing as hell. I have no idea what a cap is and how a bonus applies, if at all, to international calling. Can someone please clear this up?
Or, better yet, tell me what provider will give me the best overall rate to the USA on voice and text?

Thanks for your help.

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i want to get an unlocked cell phone that i can use back home In South Africa and i am currently in america..which phone will be the best and will I be able to use verizon on it if not how can i use the Verizon network on it or which will be the best prepaid for unlimited data?

T-Mobile is probably the best choice. AT&T is expensive. Net10 doesn't have data selection and with Virgin you can't use your unlocked phone. T-Mobile is all-rounded and answers to all the questions above! Plus, it's easy to get refill online. Try this one. CallRefill - Shop. They have special on T-Mobile prepaid refill and it's 9 cents per minute!