Verify a sign-up code

Verifies the one-time code for a verificationId. On first verification the account is created, an API key is issued, and a first phone number is provisioned. The API key is returned only once — store it immediately.

When a new number is provisioned, inboundInstruction is optional — it becomes that number's inbound voice-agent prompt, and a default greeting is used if you omit it. For an existing account (no new number provisioned) inboundInstruction is ignored.