Send an SMS/MMS

Sends an SMS (or MMS when mediaUrls is set) from one of your SMS-enabled numbers. At least one of text / mediaUrls is required. Both numbers are normalized to E.164, so from matches regardless of formatting and replies thread into the same inbox conversation.

US numbers must have an approved carrier registration (/v1/sms/registrations) before messages deliver.

Replies and delivery status arrive as webhooks, not by polling: an inbound reply fires message.received with platform: "sms", the first message of a new thread also fires conversation.started, and this message's own outcome fires message.delivered or message.failed (the latter carrying the carrier's error code).

Opted-out recipients: a send to a number that replied STOP is refused with 409, never silently dropped.

Idempotency: send an Idempotency-Key header to make retries safe: same key + same body replays the original response instead of sending a second message; same key + different body returns 422; a key still in flight returns 409.