Sending SMS messages via Twilio can fail for persistent reasons like a destination number w/o SMS service (landline or disconnected service) or a recipient who has sent a STOP request to Twilio. As of Ignition 8.1.3 these events are visible in the log as Twilio ApiException events, but there is no way to capture them programmatically. Our system needs to respond to these events in certain ways, to inform users of the failures and prompt them to reconfigure the destination numbers or send a START message to Twilio.
Ignition's Twilio module uses the TwiML API, which appears to mean that callback events from messages it sends can only be sent to the TwiML callback. Twilio does not appear to provide a way to send callback events to a separate webhook. The only current workaround would be an asynchronous script which periodically polls the Twilio API for the status of recently sent messages.
EDIT: There may be a workaround using their debug webhook you can setup here: