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That is going to depend on what Plan you are n and which country you are in. But generally, assuming you are on a Post Paid Plan and have activated International Roaming, the worst case will be pay as you go rates. If you should be able to get the Day Pass, then talking to https://livechat.telstra.com or giving +61 439 125 109 would be the way to get it activated if it's not appearing in your app.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.