There are basically three variations on how local calls are charged.
A flat rate, fee per minute and free calls as part of a package.
National Capital City Calls
The best flat rate untimed call rates are provided by Engin capped at just 10c per call.
Please note for Engin, that the person you are calling has to be in a capital city as defined here - Capital City Definition (click link). This service is not currently available for subscribers in Tasmania or the Northern Territory.
If you are calling other than capital cities then the best rate per minute is Siphone at 1.0c (min monthly plan fee $5.00).
Mobile call costs are pretty much the same for all IPSPs (around 30c a minute - about half normal phone rates). The cheapest at the moment are Pennytel - 24.706c.
Some companies offer a combined package of Broadband connection and Internet Phone service which can save you a fair amount of money.
For example Nodephone from Internode and Exetel.
Astratel also have a plan for businesses (Business Unlimited) which provides unlimited calls for $79.95 per month.
If you have found a better deal than the ones above let us know by email.