2 years ago when I got a new phone, I had to re-register in both WhatsApp and Signal.
Both send you a code over SMS. I hate giving apps SMS access, so I refuse SMS intercept permission when an app asks for it, and instead I manually enter the received code in the app.
This was possible in WhatsApp, but was not possible in Signal. It won't allow you. You must give it SMS access to read the code. So much for privacy/security/... Only app I use which didn't allow me to manually enter the SMS code.
[dead]
Hate to say it, but if you don’t trust Signal to read your texts… you’ve already lost - stick with Facebook Messenger.
I’m pretty sure it’s for security reasons, it allows the app to ensure the code comes from a correct Signal phone number and not from a third party actor. It’s hand holding yes but it’s a trade off in favor of non-technical users.