"but it also lets that phone number restore backups" - is this true? I use Authy and can use a phone number to connect a new device to my account, but to restore access to credentials on the new device I need to enter a passphrase. I don't think there's a way to restore credentials with just a phone number, but will need to check.

Edit: the following article appears to confirm my beliefs: https://support.authy.com/hc/en-us/articles/360036580453-Rec...

Without the backup password, you won't be able to restore a backup.