AT+CMNI doesn't store SMS in modem

See more:

SMS

ATCommands

C#5.0

In my SMS application, I read the COM messages with the following function :

void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    if (e.EventType == SerialData.Chars)
    {
        string t = _sp.ReadExisting();
        Debug.WriteLine("message recu : " + t);
    }
}

And I launch command in the COM port with :

public void ExecCommand(string command)
{
    _sp.DiscardOutBuffer(); //_sp is my SerialPort object
    _sp.DiscardInBuffer();

    _sp.Write(command + "\r");
}

I launch the CMTI notifications for the new messages received with :

ExecCommand("AT+CNMI=2,1,0,0,0");

And I receive the notifications (for example) :

+CMTI: "ME",0

Then, I try to read this received message with :

ExecCommand("AT+CMGR=" + pos); // pos = 0

But my only response is

OK

And the `AT+CPMS?` command :

+CPMS: "ME",0,20,"ME",0,20,"ME",0,20

The ME and SM storage are empty... how can I read the messages I receive ?