Send one SMS to a mobile number, paid per message in USDC with x402. Use it when an agent must notify a person by text. The 402 quotes the exact price for the destination and message length. No account and no API key: any wallet that pays sends in one call. An opt-out link is appended to the message, and numbers that opted out are refused. Returns an order; poll GET /v1/orders/{id} for delivery status.