Comment by px43

The list of phone numbers with signal accounts is basically public. It kind of has to be. When a new number gets added and it matches someone in your address book, your app will tell you that one of your contacts has joined. People have always had the ability to turn off that feature, but that's not what the feature request seems to be asking.

People seem to be asking for a way they can join Signal without their number showing up in the registry of Signal users. This is why it's "not possible".

edit: This may have changed today. I'm now seeing an option that lets me hide my number from the registry. This means that even someone with my phone number will not be able to message me on Signal, which seems like a good deal to me.