Telstra is the only option remote - others are nowhere near it. 3G coverage most of the way across the nullarbor for example, but 3G is going away shortly and 4G/5G do not come near the same coverage. I reckon best alternative for phone only is Iridium sat phone. Full internet really needs something like Starlink. Boosters are OK when you are marginal, but no use when you are remote. There are also other cheaper options if you only need SMS type messaging and an emergency facility.