I'd be more concerned as to why they were sending you codes, rather than what the number was for. Check your account to make sure no one is trying to hack into it.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Probably somebody else is trying to access their own account and mis-typing the number, so he is getting the responses. But as @MI5 says, take it as a warning that somebody may be trying to access your account.