Using t mobile iPhone 11 max in Australia

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

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Carol B.s point is a good one; If you have a T-Mobile Magenta Max plan it is usable in most of the world, with some minor restrictions (5 GB limit per billing period, a small per minute charge for non-Wi-Fi Calling cellular calls). But T-Mobile is also very liberal on unlocking, so call them and ask. They will unlock your phone after you have used it on their network for a relatively short time.

Have you checked with T-Mobile to see if they have a plan for you that offers coverage in Australia? I have T-Mobile in the US and can purchase various out of country plans. You can chat with a T-Mobile representative here:

https://www.t-mobile.com/cell-phone-plans/international-roaming-plans