I enabled two factor authentication with a German number. And since then I'mlocked out of my account: verification SMS never arriveAfter setting up two factor authentication, I made a quick test, and it didn'twork. 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. NowI'm locked out for more than a week, and no reply from Twitter customersupport.I then read about short and long codes. In certain countries, one can sendSMS to a short code, e.g. 40404, and the message get's posted to one'stimeline. Also one can receive tweets to the phone by SMS. In Germany, a shortcode is not available, only a long code, +491724403473. Here, receivingtweets 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, althoughthere were issues in setting that up by SMS, and my password got posted.UpdateTwitter in the end answered my original support request, which I sent on2013-09-25 CEST, i.e. prior to posting this question on Stack Exchange.Their reply: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-androidMy request (phone number and URL edited out):
Verification by SMS in country with long code? (Germany)
twitter;security;authentication;sms;multi factor auth
It is indeed true that Twitter can't send SMS in countries with a long code. Looks like there's an oversight in this system so that it's allowing SMS verification to be used even though it should require confirmation via incoming SMS.Twitter provides instructions at https://support.twitter.com/articles/20170409, but unfortunately those aren't of much use. I recommend you contact Twitter support via https://support.twitter.com/forms/sms and tell them about your case. They should be able to remove SMS verification for you. Please do read the first automatic reply carefully, as they sometimes require you to reply to it for the case to actually be opened for examination.