Wait how big is the hash of the phone number?
If it's enough bits (e.g., a full sha hash) then it's not that secure to hash at all. 10^10 or even 10^11 is just 10 or 100 billion. I can easily try all phone numbers until I find the one that matches the hash.
It maybe protects against attacks against lots of people, but it really doesn't protect an individual.
You are correct that using a hash does not protect an individual from other users discovering that they can contact them with Signal, which is to be expected because that's the purpose of this feature. If you suspect that Bob, with phone number +15555551234 has Signal installed, you can verify that by... typing Bob's phone number into your contacts list and installing Signal so you can send messages to Bob.
No matter if your phone number is six digits or sixteen, Signal uses 10 bytes (80 bits) of the hash.
What threat model has a IP be worse to leak than a phone number?