710002: the text can't be sent

Why it happens

On the POST, or later on the Message (its status failed, on its StatusCallback). reason is one of:
recipient: that number can't be texted from this line (Australian mobiles only, by default).
too_long: the text is too long.
content: it has characters that can't be sent.
invalid: the text isn't valid.
line_rate_limited: the line's own text limit is reached (the POST's own limit is 710001).
line_busy: too many texts are waiting on the line; try again shortly.
line_suspended: the line isn't active: not rented, moving to another account, or ended.
dongle_offline: the line's dongle isn't connected right now.
send_failed: the text couldn't be sent (it expired waiting, or the dongle refused it).
unknown: it isn't known whether the text went out (part of a long text may have gone): check before sending it again. It's never resent for you.
temporarily_unavailable: briefly unavailable on our side; try again shortly.

Fix the text or the number for the first four; wait and try again for the passing ones; for unknown, check with the recipient before sending again.

Besides code, message, more_info and status, the error body carries: reason. The message says the same in words.

{
  "code": 710002,
  "message": "the text can't be sent",
  "more_info": "https://docs.dongleswithwings.com/errors/710002",
  "status": 400
}