You can use Signal anonymously by using ‘Registration Lock’ with Google Voice or any other temporary number.
Make sure to enable Registration Lock IMMEDIATELY after successfully registering an available number.
(While you’re in the settings, make sure to also enable ‘Always Relay Calls’ to hide your IP from your contact.)
Registration Lock expires after 7 days of inactivity, so make sure to start Signal at least once every few days. After 7 days of inactivity, you will lose the registration lock, which will enable someone else to register and lock that number for themselves. That is why it is important to keep a few chats going inside Signal.