code: func (step *CallFlowPlayStep) MarshalJSON() ([]byte, error) {
  	data := jsonCallFlowPlayStep{}
  	data.CallFlowStepBase = step.CallFlowStepBase
  	data.Action = "play"
  	data.Options.Media = step.Media
  	return json.Marshal(data)
  }
docs: // MarshalJSON implements the json.Marshaler interface.