Is this OVO that is calling you, or is it a Debt Collection company?
As you cancelled the DD then has the account already been switched to pay-on-demand, and has the credit run-out?
(If it’s been switched from a Fixed rate tariff to standard variable then an ‘exit fee’ may have been taken for ending the fix early).
I suspect that it’s probably OVO calling you, because a Debt Collector would not usually know about the cancelled DD.
All a Debt Collertor knows is that they have been instructed to collect a debt, they don’t know or care how that debt was accrued.
if it is a Debt Collector phoning you then I have an advice post about how to stop them. It’s more about getting letters, but the advice in the first post there is relevant to phone calls from Debt Collectors too:
IF IT IS A DEBT COLLECTOR phoning you and not OVO then to start with:
Tell them that you require that all further communication must be by email or by letter and that you refuse to discuss anything on the phone, it they do ring you then inform them again and politely hang up.
(Having everything in writing avoids “you said/no I didn’t” type arguments that can arise with phone calls).