I am using `PIC18F2520` and trying to communicate with `GSM SIM900`. I am using UART interrupt to receive data from the GSM. I am saving each byte in rxData. Now most of the commands ends with response `OK`. But there are few commands which does not respond with `OK` like when we receive sms notification from GSM, GSM send following notification

     +CMTI: "SM",2

Now this response doesn't have any `OK`. How to receive data in this case. In my application I need to check for the received sms. I am using following code:

    void rx_handler(void)
    {
      rxData[index] = ReadUSART();
    
      if(<some condition>)    //condition to check for sms notification
      {
        rxFlag = 1;        //set flag to process it in main loop
        
      }
      index++;
    
    PIR1bits.RCIF==0;
    
    }

I am getting confused on how to check for the sms notification. Till now for other commands, I was setting condition for OK but this doesnt have any OK response. Please help.