[acropolite (Phil)]

The most likely cause is either a faulty Line Interface in the exchange or the jumper (interconnecting wire) may have been accidentally removed. It could also be that you have a cable fault although usually there would be some voltage present if that were the case.

In some cases it's Telstra's own (auto) provisioning equipment that disconnects the Telephony.

The cable pair is common to both the DSL and Telephony but both have different equipment attached at the exchange. The Telephony circuit is fed through a device called a DSLAM which injects the DSL signal in to the physical cable pair.

It's not uncommon to have DSL still working under severe telephony fault conditions.

There is also another possibility, if there is NBN FTTN provisioning within your area it's possible for your line to be incorrectly jumpered within the FTTN node pillar.

The reasons for such slow service restoration are low staff numbers, high workload and lack of expertise as highly trained staff are replaced with often unskilled contractors on low pay rates.