Have you dropped it at all, and I mean AT ALL ??
Do you have the latest update from Apple for iOS ?
Have you rang Optus and asked if it is something there end ??
Something like this, could very well be the Network, but if you can, I would take it in and get a Diagnostic done on it by , and see what they have to say. Its obviously still under warranty. Its not a problem we here in the forums could fix for you.
YW, just remember when you take it into Apple, be nice and courteous, and they are usually quite responsive. I have a feeling Optus will say its a Apple issue, but still it can't hurt trying, especially when you say the garble is only one way . .