Comment by imkh

There are quite a few forks that connects directly to the Signal servers, [Molly](https://github.com/mollyim/mollyim-android) being the most well-known I believe.

From my understanding, they're not a fan of it (not sure if it's officially against their TOS or not) but they don't go out of their way to stop them. At least as long as you don't use the Signal name and make it clear you're not an official app.

Even in this blog post about usernames, they clearly make sure to mention them: "This means that in about 90 days, your phone number privacy settings will be honored by everyone using an official Signal app."