A text message in a campaign can fail due to the following reasons: 

1. Twilio rejects the number
2. Bad/ landline number

3. Message not sent by Twilio
4. Message sent but never delivered

For more information on why a particular sms failed:

1. Log into your Twilio account

2. Click the cog icon at top-right and choose Sub-Accounts then select the sub-account

3. Click the circle icon with three dots in the left column and select Programmable SMS

4. Click Logs then set your timeframe and filter by Status = Failed or Status = Undelivered

5. When you find the message, hover over the question mark icon to see error code/explanation