As far as I can tell they cannot discard the salt if they want to be able to tell if you already have a 'humanID'. Without the salt, you cannot tell if the phone number already has an account without having to brute force the salt.
If they were serious about this, they should at least use scrypt or some other modern password hashing technique as salted sha512 can be computed too quickly on modern hardware.
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Why would the salt become known if it's also discarded? Do they need to re-hash the phone number a second time?
As far as I can tell they cannot discard the salt if they want to be able to tell if you already have a 'humanID'. Without the salt, you cannot tell if the phone number already has an account without having to brute force the salt.
If they were serious about this, they should at least use scrypt or some other modern password hashing technique as salted sha512 can be computed too quickly on modern hardware.