If you stream Apple music via a Telstra mobile it is data free (it doesn't matter if the subscription is direct to Apple Music). The family subscription works the same, so long as the mobile phone is on the Telstra network, it is data free streaming for music (not videos and not downloading to device for offline listening).
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.
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