When making outbound calls from Twilio, if Twilio returns an HTTP response error, this means Twilio is unable to process your request and execute your call. Your calls will not be queued up to be dialed and no charges will be made. Check out the HTTP response for the link to the error code reference to troubleshoot why your request could not be made.
If you hear "An application error has occurred" during your call, this means Twilio was able to process your request and initiate the call. Since a voice call connection was made, you will be charged for the duration of the call. Check out your Notifications log for more information on troubleshooting errors with calls.