How do I know if I can use a different SIM card in my phone?
Is your phone is able to use a different companies Sim card.
The first method is pretty simple, you can just give it a go. To do this, turn off your phone and replace the Sim card. Once you have replaced the Sim card, turn your phone back on. If your phone is able to connect to the network then it is unlocked, if it displays an error message then more than likely your phone is locked.
The second method is also pretty simple. I often find that simply calling your phone provider is the easiest way to verify if your phone is locked or unlocked. They can tell you right away and depending on your phone carrier and the plan that you're on, they can often unlock your phone right then and there (if it's not unlock already).
You can contact your phone provider while you are overseas to have your phone unlocked, but it is far easier to do this before you travel. Knowing for certain before you travel at your phone is unlocked will mean that you can feel confident that when you get there your phone will work.
For many people, if they are overseas and find that their phone is locked they find the easiest answer is simply to buy a cheap phone while they are there and to rely on Wi-Fi for their Australian phone. Buying a cheap phone while overseas is also commonly done to avoid the risk of damaging your more expensive phone while having fun on your holiday.
So next time before you travel, check and see if you can bring your phone, holidays a much better if you can keep in contact.