Message cannot be delivered this can be due to multiple reasons, one of them being a landline. Watch this video on how to remove these contacts from the workflow and incurring more errors from this contact How to remove contacts with errors from your workflow.
30007 - Message has been flagged
Message template should be updated Learn how to update your sms templates
User should verify their numbers online to reduce the chances of the number showing as spam by going to Free Caller Registry Learn more about 30007 twilio error.
Number has not been attached to the A2P campaign in Twilio. Click here to learn how to correct this.
Authentication Error
Usually means that the user’s Twilio account does not have funds and or Twilio account is suspended. To resolve this go to your Twilio account and verify there is Money in the Twilio account and there is not banner indicating Twilio account is suspended. (in order to connect your Twilio account to Offerbolt you must have a money in your Twilio account. Failure to bring your balance current in Twilio will result in Twilio suspending your account.
Website for Twilio errors codes and the description of each https://www.twilio.com/docs/api/errors