The answer depends what type of contract you are on,
Traditional pay-monthly plans have no concept of paying anything off, you agree to a single monthly fee on a rolling basis for at least the duration of your minimum-term. The user of the phone should receive a text message towards the end of your minimum-term outlining numerous options, one of which would be to change to a SIM-only plan. Ultimately however, the choice must be that of the account holder, and if nothing is requested, nothing will change.
The newer FlexPay plans have 2 separate & independent contracts, each with their own monthly payment. You have a rolling airtime-contract, which is similar to a SIM-only, this will only end if & when you explicitly request disconnection. You also have a separate fixed-term finance agreement, which will stop when the loan has been repaid.
