AgentPhone OTP Verify & Account Provisioning

Use this endpoint as the second and final step of the AgentPhone signup flow, immediately after calling the Sign Up endpoint which sends the OTP to the user's email. This is the only endpoint that atomically creates an account, provisions a phone number, and returns an API key in one step — no alternative multi-step registration flow exists.

• Invalid or expired OTP code — returns error indicating verification failed
• Invalid or unknown verification_id — returns not found error
• OTP already used or verification session expired — returns expiry error
• Account already exists for this email — returns conflict error
• Missing required fields (verification_id or OTP) — returns validation error