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You could put the phone in to Airplane mode and then off again.
You could restart your phone.
Your SIM could be faulty, so you could pop in to your nearest Telstra Store and they will replace it on the spot (or call 132200 to arrange for one to be sent out).
It could be an issue with your account, so call 132200 or use Livechat and get a consultant to have a look.