The answer very much depends whether you are on a traditional contract or a FlexPay.
If you're on a traditional contract, your airtime & device repayments are inextricably linked and not separated. You have one monthly charge covering everything, and thus one calculation for early cancellation. Months remaining × monthly cost minus a 4% (IIRC) discount. You don't receive any network service in lieu of cancellation.
If you're on a newer FlexPay plan, you have 2 separate contracts with 2 independent payments.
You have a rolling minimum-term contract for airtime that is subject to very similar rules as a traditional contract.
You then have a fixed-term finance agreement, for which I believe you can repay however much you want, whenever you want.
