(Lycamobile and Lebara, while still cheap for overseas calls, has raised its prices for local Australian calls. It stinks of anticompetitve behaviour (read: collusion). Stay away from these two)
A friend of mine introduced me to Fring, which works pretty much like Skype. Fring can be installed on most mobile devices and across various platforms. You can install Fring on Android, iPhone, and Symbian platforms. (That said, another friend sadly found that it was not suported on Android 2.1). Fring gives you the ability to make video call or voice-only call, while FringOut works on prepaid credit, which you can use to make calls to non-Fring phones.
I installed Fring on my Nokia E71 phone. I also bought FringOut credit. As my phone runs on free 3G (thanks to TPG's generous $1 mobile plan), I can make Fring calls through Wifi at home, or through 3G away from home.
Now for the cost. There is no flagfall charge. Calls to Australian landline is 2.1c per min and to Australian mobile is 9.5c per min. The only way to beat this super cheap rate is to install Fring on the other end. Fring to Fring calls are free. Forget about paid-Skype; it costs 3-4 times more than Fring for local calls.
How does this compare with VOIP?
MyNetFone charges 12.5c per min to landline (untimed and no flagfall), and 24c per min to Australian mobile (plus zero flagfall).
(For your information, FringOut calls to Malaysia landline is an incredile 1c per min, and to Malaysia mobile is 3.3c per min. As for quality of the call, it varies. I am happy enough with it, but I have yet to find out how consistently good it will be in the long run)
Updated on 30 May 2011
The quality of Fring cannot be depended upon. It is useable, but not good enough to carry on a conversation for long without wanting to