Twitter recently disclosed an “incident” in how the service handles phone numbers. The announcement declared that it had shut down “a large network of fake accounts” responsible for uploading lists of phone numbers and then using Twitter’s own API to match them to individual usernames.
So far the problem only seems to affect Twitter accounts who have a phone number associated with their account, and have “phone number discoverability” enabled in their settings. If you’re unsure, you can check the EFF’s step-by-step guide to checking your settings
here.