I bought a new Pixel 3 4 days ago and found it won't charge at all. I took it back to the Optus store and the unhelpful salesman said he could send the cable away to be checked but it would take 5 days and I would just have to put up with not being able to charge my phone in that time. When I protested he said my only option was to buy a new cable. I then had to make him register the faulty cable- he wasn't going to do it. I thought at the very least I should have been offered a replacement cable free of charge. Very unhappy with the attitude. I am now using the purchased cable with my old Samsung charging unit. It is working but it is slow.
I agree the sales man has acted pretty poorly. Leaving aside Optus waranty, consumer guarantees you it must be of merchantable quality. Also most stores will just swap over euipment if its found faulty in the first 14 days (although I don't know if Optus does this)
For a start it would have been pretty simple to plug in another cable into your phone on the spot and run a check? Any reason why you think its the cable and not the phone itself that's busted?
Regardless if it is faulty you are entitled to a free replacement. Unfortunately Optus can make the process very slow in assessing the issue.