/**
  * @param $transaction_number
  * @param null $terminal_id
  * @return string
  */
 public function getCallnumber($transaction_number, $terminal_id)
 {
     try {
         if (is_null(TerminalTransaction::find($transaction_number))) {
             return json_encode(['error' => 'You have called an invalid input.']);
         }
         $terminal_transaction = TerminalTransaction::find($transaction_number);
         $priority_queue = PriorityQueue::find($transaction_number);
         if ($terminal_transaction->time_called != 0) {
             return json_encode(['error' => 'Number ' . $priority_queue->priority_number . ' has already been called. Please call another number.']);
         } else {
             return ProcessQueue::callTransactionNumber($transaction_number, Helper::userId(), $terminal_id);
         }
     } catch (Exception $e) {
         return json_encode(['error' => $e->getMessage()]);
     }
 }