The three main mobile providers are two privately owned carriers, Optus and Vodafone, and Telstra, the former monopoly which the Federal Government is in the process of privatising. Networks vary in their long distance charges; some offer flat national rates, others have rates depending on how far you call. They also vary as to their peak and off peak charges. As a general rule, peak time is 7am to 7pm Monday to Saturday and off-peak at all other times.
When you sign up for a plan, you are usually committed for a period of 24 months so if you stay in Australia for a shorter period it can be more economic to buy pre-paid cards (more information on: www.phonecardselector.com.au). It is also the most economic solution (after Skype) for international calls. Example: a card of $20 AU provide almost 400 minutes of communications France-Australia! These cards are sold almost everywhere.
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