Calls to mobile phones are a rip-off. If I’m calling someone, it’s not my fault that they happen to be using a mobile.
Even with a VOIP provider or one of the mainstream discount landline providers, you’re likely to pay around five times as much to call a mobile in the same room as you are a landline in Australia.
DiscountDial offers mobile calls at 5p per minute, any time of day. I’ve tried it and it works. You call their 0844 566 35 35 number, then dial the mobile number. Your usual phone provider bills you the 4p per minute and DiscountDial get a revenue-share. (Number and rate updated thanks to dfear, after DiscountDial put the cost up from 4p).
It may be a promotional offer but it rocks for now.