> We use third-party services to send a registration code via SMS or voice call in order to verify that the person in possession of a given phone number actually intended to sign up for a Signal account. Simple solution, go distributed.
6M $ for that. Stop doing that. What do dictators control? Mobile phone networks and other infrastructure. And, yes, they really do go after people any way they can.
This "cost" puts people into danger. Coupling identity and operator infrastructure is a critical privacy flaw. And a costly one too apparently. If your #1 goal is to be the most private solution, this cannot be tolerated to continue to be the case. Get rid of it. Your identity should be your cryptographic key.