code: func (call Call) MarshalJSON() ([]byte, error) {
  	endedAt := ""
  	if call.EndedAt != nil {
  		endedAt = call.EndedAt.Format(time.RFC3339)
  	}
  	data := jsonCall{
  		ID:          call.ID,
  		Status:      string(call.Status),
  		Source:      call.Source,
  		Destination: call.Destination,
  		NumberID:    call.NumberID,
  		CreatedAt:   call.CreatedAt.Format(time.RFC3339),
  		UpdatedAt:   call.UpdatedAt.Format(time.RFC3339),
  		EndedAt:     endedAt,
  	}
  	return json.Marshal(data)
  }
docs: // MarshalJSON implements the json.Marshaler interface.