login problem

Did you register with your landline number? You can also change your bank details using this form.

Change bank details

I tried but it said my number is unknown. I dont know why. 
i prefer do it online.

i can fill out the form as well but is there any way to send it by email back but not by fax nor post?

send it via the contact form.

https://www.o2online.de/service/kontaktformular/

Hello ​@kayleung ,
welcome to our o2 Community 😀
You can see how to register in the following video 
Please note that the new bank details will not yet apply to the next direct debit. 
If the old bank details from which we debit are no longer available, please transfer the money manually. 
Please use the customer number on your invoice as the reason for payment and the bank details 

Hypo Vereinsbank AG
IBAN: DE 16 7002 0270 0005 7131 53
BIC: HYVEDEMMXXX

Kind Regards, 
Solveig 
 

I sent via contact form of change the bank account last month and o2 just charged my old bank account money yesterday. Please let me know when the fee will be debited from my new bank account.

Which bank details do you see in My o2?

As mentioned, i am not able to login thats why i sent the change of bank details via contact form as your colleague advised. Again as mentioned above, i tried everything and still not able to login. Please i only need to change the bank details, can you just use the form of changing bank detail that i sent via contact form?