PennyTel have an offer at the moment where you can get untimed calls to australian mobiles for a rate of 28.88 cents per call. This call rate also aplies to some other destinations that aren't already included in the flat-rate 8 cents per call. You need a PennyTel account and VoIP hardware/software to take advantage of this deal though. It can also be enabled/disabled very easily through the Pt Account interface so you can still make short calls to mobiles for the standard 12.78 cents per minute.
Just a note: make you you don't make your calls right away before activating it (it is disabled by default), or you'll get charged by the minute.