If you’re on an upfront mobile plan, international roaming isn’t compatible when travelling on a cruise ship. You’ll need to use other options such as Wi-Fi on the cruise ship, in order to use your service.
If you’re on a Telstra Mobile Plan, you may be able to use international roaming when travelling on a cruise ship. If the cruise ship turns on their satellite service, your mobile device will connect to the strongest signal which in many cases is the cruise service. If this is the case, you’ll be roaming at pay-as-you-go (PAYG) rates, even if the nearest country is eligible for day pass. As soon as this happens, you’ll be notified that you’re being charged PAYG rates.