c# - 当我使用拨号调制解调器拨打电话时，DataReceived（串行端口）事件不起作用

Question

当有人拨打我的电话号码时，我尝试获取并识别铃声，但是当我用我的调制解调器（电话号码）接听电话时，DataReceieved 事件不起作用。问题是什么？这是我的代码：

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        delegate void SetTextCallback(string text);
        private void SetText(string text)
        {
            if (this.lblStatus.InvokeRequired)
            {
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke(d, new object[] { text });
            }
            else
                this.lblStatus.Text = text;
        }


        private void sp1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {

            string rs = sp1.ReadExisting();
            MessageBox.Show(rs);
            if (rs.Contains("NMBR"))
            {
                int sx = rs.IndexOf("NMBR");
                SetText("Incomming Call From: " + rs.Substring(sx + 7, 11));

            }


        }


        private void Form1_Load(object sender, EventArgs e)
        {
            if (sp1.IsOpen)
                sp1.Close();
            sp1.Open();
            sp1.Write("AT+SCID=1;" + Environment.NewLine);
            sp1.Write("AT+FCLASS=0;" + Environment.NewLine);
            sp1.Write("AT+VCID=1;" + Environment.NewLine);
            sp1.Write("AT+PCW=0;" + Environment.NewLine);
            lblStatus.Text = "Ready...";
        }


}

}