On the Google Pixel it's called "Backup Calling", and if you enable it it will try using the data of your other (non-roaming) SIM to call if your Telstra SIM is roaming or has no signal. This is the easiest to set up.
On Samsung phones it's a bit janky. If you have Auto Data Switching and Wifi Calling enabled then Backup Calling is enabled, but will only kick in if your Telstra SIM has no signal (not when roaming). This is a bit annoying, but I find a workaround is to use the "Network Operators" menu to select a unsupported network, thus forcing a no signal condition, thereby causing Backup Calling to be enabled.
If working it'll show something like this (the phone with SIM 2 icon). This means my SIM 1 is Wifi calling through the data connection of SIM 2.
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I don't personally have an iPhone, but it works similarly to the Samsung situation described above as I understand. You need to enable "Allow Cellular Data Switching" then set your Telstra SIM to a no signal state whilst your other SIM has a data connection (probably by using the network operators menu and selecting an unsupported network too). In your iPhone control center for your carriers it should then say something like "Telstra (using Cellular Data)".
Maybe I'll write up a dedicated thread in this forum about it.