signal is uploading contact identifiers
The "signal" messenger is criticized for uploading hashes of the user's contacts from the address book database, and revealing the users social graph. (Hashes are also easy to calculate for the ranges of phone numbers.) https://en.wikipedia.org/wiki/Signal_(software)#Contact_discovery
The user should at least be educated that signal is uploading contact identifiers to central servers.
To fix the issue, maybe find an alternative to use, for example an email-chat messenger (like https://delta.chat, or the new signal based app version https://github.com/deltachat/deltachat-android-ii/releases, which do not use central servers at all, besides the email servers).