Optus phone trade up?

So I'm coming up to my 12 month phone trade up and want the IPhone X but I'd still like to keep my current phone and won't be able to afford to pay the rest of the phone there and then. How does it work do they add extra $50 or whatever onto my bill to pay out the phone or do they charge me the full amount I owe on the phone to the bill? Someone please explain Australian Optus users hopefully

You WANT two phones- you PAY for two phones. However they expect you to do this is up to THEM. If they want payment in full, then you can always stick it on a credit card - but then, you are paying MORE, since there's interest. Something tells me that if you can't figure this out, you are going to be in a deep mess.