Store SMS message in variable

I'm having trouble saving an SMS message to a variable. The idea is to send an SMS with a command and later use a conditional operator to activate or deactivate a relay.

But I can not save the SMS message. Whenever I try to save it, it assumes the value of:

" ÿ " .

I'm using a sim900 GSM module with Arduino.

Here is the code I'm trying to use:

#include <SoftwareSerial.h>
SoftwareSerial cell(2,3); 
void setup()
{
    cell.begin(19200);
    cell.println("AT+CMGF=1"); 
    delay(200);
    cell.println("AT+CNMI=3,2,0,0");
    delay(200);
}
void loop() 
{
    if(cell.available() >0)    // se o shild resceber uma msg
    {          
        delay(10);
        msg=cell.read();     // salve ela na variavel  msg
        Serial.println(msg);// mostra o valor do "msg" NESSE ponto ela esta valendo ÿ 
        if (msg=='a')     // se o primeiro caracter for "a"
        {
            delay(10);
            msg=cell.read();
            if (msg=='0')   // se o segundo caracter for "0"
            {
                delay(10);
                msg=cell.read();
                if (msg=='e')   // se o terceiro caracter for "e"
                {
                }
            }
        }
    }
}

When I print the message on the serial, its value is ÿ , why this?

I researched and found that before telling Arduino to store the SMS, we have to tell him to "listen" to the serial that receives the message and this is related to:

gsm.listen(); 

But I did not understand the relationship very well.