By providing Signal with any phone number at which you can receive an SMS or text message, you can register a Signal account at that other phone number. For example, you can create a pseudonymous Google account, register a Google Voice VoIP number, and use that as your Signal number. Or you can even use a free throw-away SMS account and use that number when you sign up for your Signal account instead of your real phone number. The Signal service will happily send the throw-away number a text message with the verification code, letting you complete the account sign-up process.