Enter the code the carrier texted to confirm a person's US texting registration. Use it when the verification shows `confirmation: "code"`. On success the campaign is usually filed at once. It waits when your credit does not cover its fee (`waiting_for_credit` is true until you add credit; `credit_needed_cents` says how much). It also waits while another check of it is running or recently failed, or another process holds it; a later check files it. Poll GET /verifications/{id} until `approved`. A wrong or expired code is 422 on `code`; after five wrong codes, 429 until the carrier sends a new code (once a day). 409 when there is nothing to confirm.