Yes, you can do this even if you don't have a bill due, your account just goes into credit and when the bill is issued, the charges eat into the credit on the account.
Only thing to be careful of is depending on how you pay and your plan, there can be fee's for paying (like credit card processing fee etc). I'd double check that first to be safe 🙂
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I do not represent Optus. The views, opinions and advice expressed in my posts are my own
