When I was with Optus I would log the faults with them and they would log it with Telstra. Turn around was usually alright, especially if it was what I’d call a real hard fault (like water on the line or broken line) which they could see from helpdesk.
I then moved to Exetel who resold the Optus connection… so then it was Exetel → Optus → Telstra and it took longer but generally worked.
The real problem was intermittent faults because you ended up playing a game of chinese whispers.
In the end our line was so flakey we chose to move to Telstra directly so we kick them 1st hand to fix things and we got a much better response (again for those harder to diagnose problems).
What I say to most people who ask is this:
If everything works well and you have no issues with the line then all providers are viable and it’s down to back end contention ratios and cost.
If the line is a little dodgy, then I strongly recommend you deal with whoever owns the infrastructure (so Telstra).
NBN was set to change all that with a common provider to state of the art fibre infrastructure… now… well it’s still supposed to be a common provider in that NBNCo will own it all/lease/control it all (not sure which anymore) but how it will all actually work is anyone’s guess.
From what I understand Telstra have the maintenance contract so it will be RSP → NBN Co → Telstra to fix anything that’s FTTN in the future… but that’s off track for this conversation.