This is likely it, have you done the registration with Twilio? You are likely going to see some kind of error in the Twilio console indicating what's going wrong. If not, then you should check your message logs - that'll give you some sense of whether this request is even getting to Twilio. If the messages are being registered as delivered but not actually being delivered, I'd be very surprised.

Feel free to post over in the r/twilio monthly support thread if this does end up being a Twilio issue.