Without more info its hard to work out the cause.
You should have received an email with the contract documentation, this would give you the complete costings.
Have you received a bill where the handset charge was just $7?
Have you changed plans or recontracted to a different plan?
I know that lease plans are normally around $10 a month cheaper, did you sign up for a lease plan but were put on a standard plan?
I guess the plan is no loinger on the Optus website, what handset was it for?
On my bill it shows the handset charge and how many reamins, then in a different section it shows that I receive a credit for part of my handset - does your monthly bill show this?