0011 1 then the number. I set up a $5 overseas package with "3" before my girlfriend moved over here, we talked & texted shyteloads & I never went over my cap.
Pretty sure you're meant to just put +(country code) instead of the first 0 in the mobile number. SO to call an Australian mobile you would put +61 instead of the first zero.
In general, calling overseas via mobile is pretty expensive. You can get global roaming plans that will probably make it cheaper. Texting overseas is only about 50 cents more than usual.