on 23-01-2020 09:05
Be advised that a direct debit is the preferred way to pay and the responsibility will be on you to make payments on time, but if you wish to cancel, you need to contact your bank to end it.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)
on 23-01-2020 11:33
You can do it using online banking but be aware that O2 will disconnect the phone for any payment not made within couple of days of the due date. There would be a reconnection/late fee and it could be another couple of days before your phone can be used again. There is nothing wrong with making card payments as long as you don't forget.