calls.go
func (c *Call) EndedUnsuccessfully() bool {
	// https://www.twilio.com/docs/api/rest/call#call-status-values
	switch c.Status {
	case StatusCanceled, StatusFailed, StatusBusy, StatusNoAnswer:
		return true
	default:
		return false
	}
}
func (c *Call) EndedUnsuccessfully() bool {
	// https://www.twilio.com/docs/api/rest/call#call-status-values
	switch c.Status {
	case StatusCanceled, StatusFailed, StatusBusy, StatusNoAnswer:
		return true
	default:
		return false
	}
}
// EndedUnsuccessfully returns true if the Call has reached a terminal state
// and that state isn't "completed".