Normally there will be a number of phone service providers that specialise in long distance calls. Some of them you can specify the country that you wish to dial regularly so you can get additional discounts on those calls (at the expense of calls to other countries usually). Her best bet would be to ring around to all the bigger telcos and see what they have to offer (AAPT, telstra, optus, primus telecoms are those who come to mind off hand).
I would say to shop around and try and find one of the "card for service" providers. They can be as cheap as 10c per minute. You buy a card for say $20 and then dial a number, put in your pin code and card # and then dial the call. Everywhere from newsagents to newsstands sell them.