OTP — WhatsApp primary, SMS fallback
WhatsApp Cloud API is tried first; if it is not ready or fails, Message Central SMS is used.
Channel selection
Decision logic
- If WhatsApp is enabled & ready → try WhatsApp (
wa:prefixed verification id). - If WhatsApp send fails → fall back to Message Central SMS.
- If WhatsApp not configured → use Message Central SMS directly (current default).
Cost note
Per SYSTEM_LITERACY.md §18, SMS OTP is currently cheaper for utility auth in India, so SMS stays the default OTP channel. WhatsApp is used where it is not more expensive (e.g. marketing/order updates).