Register a callback URL that we will post document events to. Note: a document_signed event is not emitted solely because a pre-signed template signature/initials was copied as document content, and event.related_signer only identifies a real recipient signing action. Two additional event types cover SMS delivery outcomes: document_sms_failed fires when an SMS reaches a terminal failure (undelivered or failed), and document_sms_opted_out fires when a phone number is newly added to the SMS suppression list. Both carry event.related_recipient (id, name, email when present, phone_number_masked, and delivery_method) instead of event.related_signer, and each recipient in data.object.recipients[] whose delivery method includes SMS carries a nested sms object (phone_number_masked, send_sms, send_sms_delay, delivery_status, message_type, failed, failed_details). Email-only recipients keep their existing payload shape unchanged. Only webhooks registered on API-type hooks receive these two events.