Changing direct debit account details

[PeeTeeGee]

Hi, I'm trying to change the account that my O2 direct debit is taken from. I'm doing it on the app but when I input my new bank account details it says they're not valid. They are 100% valid and correct. Tried using O2 chat which is utterly abysmal and can't recognise a simple question, now sat on hold for 20 mins on the phone just to get to speak to someone. Appalling customer service on every count so far. Primarily can anyone advise why th esystem isn't recognising perfectly good bank account details? EDIT: forgot to mention I've also tried the 'download direct debit mandate' option and that just hangs and won't load in the app. 

[MI5 · Accepted solution]

@PeeTeeGee 

You need to contact payment management
To pay bills, ask for payment extensions etc, you need to contact the Payment Management Team.
You can reach the Payment Management team on 0800 902 0217.
The opening times are:
Monday to Friday – 8am to 8pm
Saturday – 8am to 6pm
Sunday – 9am to 6pm
https://www.o2.co.uk/help/account-and-billing/payment-support
Best time to call is around 8.00am