SMS registration action required event

Fired when an SMS registration starts waiting on its owner. reason says why: changes_requested (we need answers to some points, before submission or to fix a carrier rejection; message is the request, answer with POST /v1/sms/registrations/{id}/respond), otp_required (a sole-proprietor brand needs the code texted to its mobile, submit it with POST /v1/sms/registrations/{id}/verify-otp) or carrier_info_required (the toll-free carrier asked for more information; the request expires after 7 days). A carrier rejection alone does not fire it: we handle the fix, see sms.registration.status_updated. Fires once per new request (not on follow-up messages) and once per OTP or carrier request.