signalapp/registration-service ? reverse-engineered prompt

Build me a backend service for phone number registration and verification, like what a messaging app would use when someone signs up with their number. I want clients to be able to request a verification code and then submit that code to confirm they control the number. It should support sending codes by SMS or by voice call, and it should be able to work with more than one provider so the service can choose the best sender for each request.

Please make the API gRPC based, keep the input cleaned up and validated, and have the main service handle the overall registration flow. Store verification sessions somewhere configurable so we can track active codes or provider session references. Make the sender choice configurable too, not hardcoded.

It should be driven by config for production use, including cloud storage and analytics hooks where needed, and it should support test or fictitious phone numbers for safe development. Use the existing project structure and conventions, and look up current docs online if you need to.