Will I be charged if Twilio encounters an error when sending an SMS?
No. If Twilio returns an HTTP response error, this means that Twilio is unable to process your request and thus you will not be charged. Your request may be rejected due to a variety of reasons. Check out the error and warning code reference page to learn more.
Note that any message marked as ‘sent’ in your SMS logs will be billed to your account. This is an indication that Twilio has successfully sent the message to our carrier partner for delivery. In some cases SMS messages may be sent from Twilio but are not received at the end user’s handset. Check out our SMS troubleshooting guide for detailed troubleshooting steps.