What would be the cheapest way to call an Australian mobile ?

Dewijones

And about how much does it cost ?
Dewi

DullGreyGuy

Do you and they have data? In which case use your preferred internet voice app (eg WhatsApp, VOIP, FaceTime etc) and it will be free.

Is this a one off or a regular thing?

If they dont have data you can use VOIP still but then you are looking at around 12p/min

There are always call forwarding services but their pricing is always a little smoke and mirrors so you need to look into it closely before falling for the claimed headline rate

ballyblack

on Yolla wifi calling

Australia is only .03 a minute

https://yollacalls.com/

Neil49

Lebara contracts include 100 minutes of calls per month to Australia.

I'm currently using their 1p per month contract. Keep an eye out for future similar cheap deals. 

Alternatively, WhatsApp is a very good option.