[Mediterraneo10]

In many countries (an ever-expanding list), you cannot purchase a SIM card without showing ID, and a copy of your ID is made and sent to the state authorities. That is, mobile numbers are always connected with your identity, you cannot have an anonymous phone number. Consequently, the state can easily determine which of its citizens are using Signal.

While communications on Signal are end-to-end encrypted, in authoritarian states merely using a secure messenger can draw police suspicion.