C# 在asp.net中发送sms后，GSM调制解调器断开连接

我正在尝试使用

asp.netmvc4

开发一个短信发送系统，我正在使用华为的USB 3G调制解调器发送短信。问题是，每次我点击发送，我的调制解调器就会断开连接。这是我发短信的代码

public ActionResult SendSms(int RentId=0)
{
    AmountInfo BillTable = db.FlatInfoes.Find(RentId);
    var GetName = db.FlatInfoes.Where(a => a.serial.Equals(BillTable.serial)).FirstOrDefault();
    string getName = GetName.name;
    int getAmount1 = GetName.amount1;
    int getAmount2 = GetName.amount2;
    int getAmount3 = GetName.amount3;
    int getAmount4 = GetName.amount4;
    int getAmount5 = GetName.amount5;
    int getAmount6 = GetName.amount6;
    string getNumber = GetName.phone;
    try
    {
        SP.PortName = "COM8";
        SP.Open();
        string ph_no;
        string the_no = getNumber;
        string txt_msg = "Name: " + getName + "   Amount1: " + getAmount1 + "   Amount2: " + getAmount2 + "   Amount3: " + getAmount3 + "   Amount4: " + getAmount4 + "   Amount5: " + getAmount5 + "   Amount6: " + getAmount6;
        ph_no = Char.ConvertFromUtf32(34) + the_no + char.ConvertFromUtf32(34);
        SP.Write("AT+CMGF=1" + Char.ConvertFromUtf32(13));
        SP.Write("AT+CMGS=" + ph_no + Char.ConvertFromUtf32(13));
        SP.Write(txt_msg + Char.ConvertFromUtf32(26) + Char.ConvertFromUtf32(13));
        SP.Close();
    }
    catch (Exception ex)
    {
        throw ex;
    }

    return RedirectToAction("RentManager");
}
}

我的代码或AT命令中是否有错误？我如何解决这个问题？我非常需要这个。Tnx