http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/ur/module/how-to-use-signal
"Who Can Find Me By Number," controls whether someone can type your phone number into the search bar to find you, or pull up your phone number through their address book. This is set to "Everybody" by default. If you set it to "Nobody," then even people who do have your number in their address book will not be able to easily find you on Signal.