Of course when the modem is on you can't be sure that it's not doing that but that's what we have laws for.
https://www.thalesgroup.com/en/markets/digital-identity-and-...
But the sim card doesn't have access to the car telemetry. Probably even the whole baseband module doesn't. It just gets that data to transmit when an accident happens.
The worst you could do by hacking the SIM is to make the modem send nearby cell data somewhere. Which is serious enough because the rough location can be derived from it. But it's a far cry from what these manufacturers collect.
But I guess you mean on the car side like two modems? Yeah that would be nice, or at least to mandate the option to turn all manufacturer telemetry completely off. The EU never bothered to do this for computers and phones either though.
No need of having always on connection to the network, only enable on emergencies. No remote hacking of SIM/base band possible at random times. Mechanic or user can check battery periodically, replace if low, just like wiper fluid. Car could even cut all electric systems after deploying the integrated V-16 system.
Car manufacturers deciding to make their ECall implementations complex and privacy invading was their choice.
https://en.m.wikipedia.org/wiki/ECall