Send an SMS from the inbox number

Requires message_send. POST returns HTTP 409 outbound_not_registered. HTTP 409 phone_number_not_found when the inbox has no active number. A recipient who replied STOP returns HTTP 409 recipient_opted_out until they reply START. The first message to a recipient adds an opt-out footer. HELP and STOP as the whole body return HTTP 400 reserved_keyword. SHAFT content returns HTTP 400 shaft_blocked. A URL shortener returns HTTP 400 short_link. The body, including the footer, must be 320 characters or fewer (HTTP 400 body_too_long). Outbound segments past the monthly allowance return HTTP 402 plan_limit_exceeded with resource sms_segments. Starter allowance is 100 segments per active number. Pro allowance is 1,500 plus 100 for each active number past the 3 included numbers. Purchased segment packs add 500 each when SMS billing is on. A paused inbox is HTTP 403 inbox_paused. The stored message has channel sms. A later Twilio status callback sets status to delivered, undelivered, or failed, and delivery_events carries the numeric error code. client_id and Idempotency-Key follow the send rules. HTTP 404 not_found when SMS is off.