For some reason I have been unable to make calls whilst in the USA with my Optus Prepaid. I just logged onto Zoo, and it's telling me that roaming was turned off. Which is a surprise to me because AutoRoam is normally turned on by default, and had been like that for as long as I could remember (i.e. even when I went overseas last time, I didn't bother turning it on explicitly, and it just worked).
No expletive you could possibly think of could exceed the one that I was screaming inside when I found this out.
Of course, all of this doesn't explain how I could send SMS without any problem!