I enabled two factor authentication with a German number. And since then I'm locked out of my account: verification SMS never arrive
After setting up two factor authentication, I made a quick test, and it didn't work. As I still had the settings page open, I created the screen shot below (phone number edited out). Then I closed that page and hoped for the best. Now I'm locked out for more than a week, and no reply from Twitter customer support.
I then read about short and long codes. In certain countries, one can send SMS to a short code, e.g. 40404, and the message get's posted to one's timeline. Also one can receive tweets to the phone by SMS. In Germany, a short code is not available, only a long code, +491724403473. Here, receiving tweets by SMS from Twitter does not work. That makes me to wonder:
Could it be that Twitter does not send verification SMS in countries with long codes? Could that be the reason why I'm locked out?
By the way, using the long code, I can still post to my timeline, although there were issues in setting that up by SMS, and my password got posted.
Twitter in the end answered my original support request, which I sent on 2013-09-25 CEST, i.e. prior to posting this question on Stack Exchange.
Thank you for contacting us about login verification!
It looks like you are using a one-way long code or an unsupported carrier to add your phone to Twitter. By one-way we mean you can post Tweets to Twitter, however you will not receive anything from us (read more about it here - https://support.twitter.com/articles/87284-tweeting-with-an-unsupported-carrier). This means that you are unable to receive login verification text messages from Twitter at this time.
Since I have confirmed you as the owner of this account, I have updated your settings to disable SMS login verification. You should now be able to log in to your account as before.
Please note that Twitter for iOS or Android users should still be able to enroll in login verification from within the Twitter app. You can read more about this feature here: https://support.twitter.com/articles/20170388-using-login-verification#login-verification-for-ios-android
My request (phone number and URL edited out):