When you register with Signal, they do know your phone number. This gives them the information that "the person who owns this phone number is registered with our service." That is not linked in any way to what leaves the client when you send a message because, I cannot stress this enough, you don't send your phone number or identify yourself in any way to Signal's servers when you send a message. Please take a look at the client source code yourself.
I won't be replying anymore, have a great one! There are better things to use my PhD in cryptography for.