Maybe this is a workaround for you. I have a few generally inactive Twitter accounts, and only the last one (chronologically) I entered a phone number for. Maybe it's a recent thing for Twitter to require a phone number?
Anyway, I checked the notes I kept when setting them up. The phone number I used had two extra digits at the end. You know that any extra digits dialled are superfluous, so it only requires Twitter to not unreasonably limit the length of your phone number. I used phone number like this: +6141212345678 (example). If you can sign up like that, and the +43123123456 (example) is correct, then it doesn't matter about the 78.
If you have your user name and password for your other Twitter accounts then you can check when you log in. Otherwise forget about it, because they may have been from someone else, or some other people, who used to have you phone number!
That's the only reason I can think why I must have had to add extra digits to my phone number when creating the Twitter account, as I don't remember ever giving Twitter a phone number for my early accounts, although, maybe I did, as my extra digits were 05, and this was the FIFTH Twitter account I had ever created.