We have become aware of an issue related to one of our support forms, which is used by account holders to contact Twitter about issues with their account. We began working to resolve the issue on November 15 and it was fixed by November 16. This could be used to discover the country code of people’s phone numbers if they had one associated with their Twitter account, as well as whether or not their account had been locked by Twitter.

https://help.twitter.com/en/support-form