I suspect they're not with one specific carrier but have a data eSIM on a contract with a global supplier that has individual contracts with each provider in the country so it's hopping around with whomever has coverage. I don't know this for a fact though, simply basing this on the car seemingly having decent LTE coverage in some parkings where my mobile Telstra hotspot doesn't have any or only marginal signal, yet having good signal in locations where only Telstra serves. So by the logic of deduction, it must be with Telstra, but likely not only with Telstra.
That’s not how eSIMs work. eSIM just means that a physical SIM is no longer required, but the functionality, security and network access controls are implemented in software instead. To authenticate on a network, an eSIM still needs to represent a valid account with the network provider, and every SIM has a defined “Home Network” e.g. Telstra to which it will always attempt to connect to first.
An eSIM cannot randomly connect to some other network if it wants to. To connect to another service provider would require a roaming agreement to be in place between the home network provider and the roaming provider. And those agreements, where they exist, are very specific about where this can happen. They are geography based, not time of day based. Typically the geographic component is obvious - e.g. your device is in a different country, cannot see its Home Network any more, and so attempts to connect to some other network it sees (provided that network is not in the forbidden list on the SIM). If an authentication attempt with another network fails, that network is added to the forbidden list on the SIM, and the SIM will never attempt to authenticate on that network again.
The only roaming agreement I’ve heard of between Australian operators is between Optus and Vodafone in some rural areas where Optus has coverage and Vodafone doesn’t. To roam from one network to another, the device the SIM is in needs to fall out of coverage of the Home network and authenticate on the roaming network. That is completely under the control of the network operators involved, neither Tesla nor a hypothetical “global supplier” can influence that or dictate the terms under which it will or won’t happen.
I assume that in Australia, every Telstra SIM would have Optus and Vodafone AU listed as “forbidden” networks on their SIM meaning a Telstra phone will never, ever attempt to authenticate on the Optus or Vodafone networks. But Optus and Vodafone would not have each other on the forbidden list otherwise roaming couldn’t happen. But both would have Telstra as forbidden.
Emergency (000) calls are the only exception because no authentication is done to make an emergency call, so the phone can make one on any network it sees.
