Hi @dobey. I hit the same problem than you and found out that I need to:
Add the number to the UT contact app first In the Signal app, tap on "Import contact" Select all contact (even those who doesn't have Signal, that's the quickest way) Import all contacts: only those known by the Signal server will be imported Your new contact is there, and you can write to him. If it doesn't receive the message, reset the security keys