AT commands and modems using serial port

Hi, I wrote some code for two modems: one, must send some string and another must answer the call and receive the string. My receiver modem is a D-Link voice-modem which has COM port, and the sender modem is a NetForce usb2.0 data-modem. Two modems are connected to two different computers and each one has a phone line. I first run the receiver's code and then the sender's code. Here's the code for each of them and log files:

The receiver modem's code:

        SerialPort sp = new SerialPort("COM1");

        private void Form1_Load(object sender, EventArgs e)
        {
            sp.NewLine = "\r\n";
            sp.Parity = Parity.None;
            sp.DataBits = 8;
            sp.StopBits = StopBits.One;
            sp.DtrEnable = true;
            sp.WriteBufferSize = 1024;

            sp.Open();
            sp.WriteLine("ATE0");            
            sp.BaseStream.Flush();            
            sp.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(sp_DataReceived);
        }

        void sp_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            string str;           
            str = sp.ReadLine();
            if (str == "RING")
            {
                sp.WriteLine("ATA");
                sp.BaseStream.Flush();
            }
            System.IO.File.AppendAllText("c:\\log_receiver.txt", str + "\r\n");
        }

        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            sp.Close();
        }

and its log file(log_receiver.txt):

OK

RING

CONNECT 9600

And sender's code:

        SerialPort sp = new SerialPort("COM8");

        private void Form1_Load(object sender, EventArgs e)
        {
            sp.NewLine = "\r\n";
            sp.Parity = Parity.None;
            sp.DataBits = 8;
            sp.StopBits = StopBits.One;
            sp.DtrEnable = true;
            sp.WriteBufferSize = 1024;

            sp.Open();
            sp.WriteLine("ATE0");
            sp.WriteLine("ATDT6632");      
            sp.BaseStream.Flush();
            sp.WriteLine("My string to be sent");
            sp.BaseStream.Flush();
            sp.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(sp_DataReceived);
        }

        void sp_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            string str;            
            str = sp.ReadLine();
            File.AppendAllText("C:\\log_sender.txt", str + "\r\n");
        }

        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            sp.Close();
        }

and the sender's log file(log_sender.txt);
ATE0
OK

NO CARRIER

I have read all the commands of the Hayes AT commands, but there were no tutorial about how to use the commands(I know that using commands may differ in some modems). I wanted to know that where's the problem of my code and does anyone have an idea about using AT commands in order to send and receive data between modems?