O2 keep sending verification codes to original sim card phone number but I kept my old number

[Elsart]

I've been on live chat but they couldn't understand my query. I changed to my old number several months ago. No problems at all except when try to get on my02

[MI5]

@Elsart 

This is a customer to customer community, not O2, so we cannot access your account, sorry.
You need to contact O2.
Message them on Social Media. Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG) or give them a call on 202 or 0344 809 0202 (contract) 4445 or 0344 8090222 (PAYG)

Try online https://mymobile.o2.co.uk/ as the app can be flaky and sometimes needs to be deleted and reinstalled.