Hi there.... basically there are two goods options.

1. Internal calls
Do your phones use a SIM card, or are they like most US phones that don't have one? Here we mainly use SIM cards in our phones, and in your case rather than buying a prepaid phone like one could at Walmart, here you can buy a prepaid SIM card and put it in your phone. You can get credit and the prepaid SIMs at chain stores or 7-11s and places such as this.

This is not to say you can't still buy a prepaid phone & sim together here, they sell them at k-mart etc, but if your phones take a sim card it is cheaper just to buy that!

2. Calling US
I'd suggest you got a calling card at a store like 7-11 or one of the shops in an area like chinatown in the big cities. The smaller shops in places like chinatown tend to know which card is best for calling where, wheras at 7-11 you will have to pick it yourself! Don't be fooled into buying one of these cards from a big name like Verizon or AT&T, or Telstra here... as the rates are ridiculously high still. You can get a cheap brand $5-10 card and get far far more minutes.

Hope this helps..