The old network, whilst depreciated and with a near zero marginal utility, has a residual value: as scrap metal - copper, lead and a little tin.
But it comes at a price: it's mixed in with a bunch of other toxic material Perhaps enough cadmium and other heavy metals to be dangerous.
"Back in the day" people would burn the insulation off old cable in open fires - releasing nasties into the environment. When you have 100,000's of tonnes of old cable to deal with, that can't be hidden.
Telstra is an ethical and socially responsible organisation - its not about to release carcinogen's in smoke or heavy metals in waste streams. Lets hope they choose to process the scrap in Australia with high Environmental standards, not send the problem to the Third World and bestow lingering deaths on future generations.
It'd be even better if Telstra could sell it's exchanges as working units to economically developing countries. They've been well maintained and will probably last many more decades. That's a lot better than grinding up the boards for scrap copper, gold, maybe even tantalum.
There are many types of cable in use in the telephone network, some in lead sheaths, some paper insulation, some plastic (PVC?) insulation and some filled with a waterproof jelly that would take specialist knowledge to remove. To you use acids to leach the copper, heat to remove the casings or strong solvents, or all them?
High temperature sealed furnaces, anyone? That'd be a project for the CSIRO!
What's sure is that after extracting the valuable scrap, there will be toxic residues that will need to be safely dealt with. And another billion or two of windfall income for Telstra, making shareholders a little happier every day.