So they said Payment $55 then Credit -$55 = $0 you paid? Sounds like you weren't paying off the phone this time. You should have got another $10 or so a month off the bill as well though. Seems like an awkward lease but not lease arrangement.
Anyway, if you're having no luck talking to Optus then I suggest you write a simple letter detailing the issue and how you want it resolved to the official complaints department.
Customer Relations Group
PO Box 306
SALISBURY SOUTH
SA 5106
You'll get an official response in a few weeks with an official complaint number etc. If not happy with the reply (or you get none) then you can submit that info to the TIO (ombudsman) who will then issue a please explain to Optus. The process takes a bit of time but you don't have to do much in the mean time.