[b3lvedere]

According to Signal's FAQ:

Signal periodically sends truncated cryptographically hashed phone numbers for contact discovery. Names are never transmitted, and the information is not stored on the servers. The server responds with the contacts that are Signal users and then immediately discards this information. Your phone now knows which of your contacts is a Signal user and notifies you if your contact just started using Signal.

However; I don't know if their server is peer-reviewed on a regulary basis, nor do i know if the software (besides the used ciphers) is also peer-reviewed. Can't find anything on it.