Sends an SMS — or an MMS, when media is supplied — from one of your account's phone numbers to one or more recipients. The number in fromNumber must be one you have access to; a number you do not is rejected with 403. Recipients are converted to E.164 and de-duplicated before dispatch, and any that cannot be converted are dropped. Sending is billable and cannot be undone; usage is metered against the account that owns the sending number, and the segment count is provisional until the delivery receipt arrives.