Signal is a "private" instant messenger.
You cannot create an account in Signal without giving it your phone number.
Signal claims to be a Peer-To-Peer messenger. But this is not true because Signal stores your phone number in the servers.1
The Signal legal terms3 claims they have the
decryption keys:
"Additional technical information is stored on our servers,
including randomly generated authentication tokens, keys, push
tokens, and other material that is necessary to establish
calls and transmit messages. Signal limits this additional
technical information to the minimum required to operate the
Services."
It also says it would give info to the government and law
enforcement agencies:
Third Parties. We work with third parties to provide some of our
Services. For example, our Third-Party Providers send a
verification code to your phone number when you register for our
Services. These providers are bound by their Privacy Policies to
safeguard that information. If you use other Third-Party
Services like YouTube, Spotify, Giphy, etc. in connection with
our Services, their Terms and Privacy Policies govern your use
of those services.
Please note the "Services like youtube, spotify, giphy". Those
sites are in the Signal source code4, so they can spy
on you.
Then, we have this:
Other instances where Signal may need to share your data
To meet any applicable law, regulation, legal process or enforceable governmental request.
To enforce applicable Terms, including investigation of potential violations.
To detect, prevent, or otherwise address fraud, security, or technical issues.
To protect against harm to the rights, property, or safety of Signal, our users, or the public as required or permitted by law.
1: A Look Into Signal’s Encrypted Profiles