I'm not sure what is going on but basically my data usually works fine and I can call and receive calls but recently when I try and turn on my data nothing happens, even though my mobile data says it is on, it doesn't pop up with 4G or 3G and I can't seem to find any setting to fix it. I also found that whenever I try to call people back it always says "call failed" and no matter how many times I try. iMessage and text message are also working fine.
Have you tried a Network Reset?
Failing that, it might be worth popping into an Optus store and asking for a free SIM replacement.