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As your line rental is paid 30 days in advance and you have already paid for the final month, you will be able to cancel 30 days before the end of contract. Checking your My Vodafone Application will give your contract end date.
A good way of cancelling is to Text PAC to 65075 to receive a PAC Code, this way you will be able to keep the same number on an alternative network and when the PAC is used, this will generate the final bill. It is recommended to keep the direct debit open until the final 0.00 bill is received as your confirmation the account is fully closed.
Further info for you here: How do I cancel my Pay monthly contract?