Verify a sign-up code

Verifies the one-time code for a verificationId. This endpoint has two possible outcomes, and you must branch on the response body:

• Signing in to an existing account — the account and API key are returned immediately. The API key is returned only once — store it immediately.
• Registering a new account — the email is now verified, but an account is not created yet. The response carries a registrationId instead. Registering requires a verified phone number: continue with Register a phone number, then Verify a phone number, which creates the account and returns the same shape as the sign-in outcome.

In short: if the response contains registrationId, the flow is not finished. A new number's inbound voice-agent prompt is always the default at creation; change it afterwards with Update a phone number.