From eff.org
The bad news is that Twitter has disclosed a failure to protect users’ phone numbers, again. The good news is that Twitter users can take steps to protect themselves.
Earlier this week, Twitter announced it had discovered and shut down “a large network of fake accounts” that were uploading large numbers of phone numbers and using tools in Twitter’s API to match them to individual usernames. This type of activity can be used to build a reverse-lookup tool, to find the phone number associated with a given username.