It is possible for the phone companies to improve the mobile service in the outback, if they are pushed to do it. Satellite feeds to towns are one option.
Birdsville gets first satellite-fed 3G service - ABC News (Australian Broadcasting Corporation)
Part of the problem is the system adopted by the Hawke Government which made each phone company erect its own network, so you get duplication of networks at a higher cost. There is another option of having only one national network and letting multiple phone companies pay licence fees to operate on that network, making it self-funding. Theoretically, we could have had a much greater coverage for the same cost under that model. However, those that are supposedly wiser than us rejected the national network model in favour of a competition model, which has had exactly the negative effects which were predicted - duplicated costs and reduced coverage area. It didn't have to be that way.