如何在C＃中使用AT后发送DTMF命令？

问题描述

我需要从USB调制解调器拨打一个号码，在另一方接听电话后我需要发送几个DTMF​​命令。我试图做这个代码 - 但是当我尝试发送DTMF时退出调用：

I need to dial a number from USB modem, after other side answer the call I need to send several DTMF commands. I have tried to do this code - But it exit the call when I try to send DTMF:

 SerialPort port = new SerialPort("COM7", 115200);
        port.Open();
        port.ReadTimeout = 1000000;
        port.NewLine = "\r";
        port.WriteLine("ATZ"); // reset the modem
        //port.ReadTo("OK\r\n"); // wait for "OK" from modem

        port.WriteLine("ATDT 123456789"); // dial number with dialtone
        port.WriteLine("AT+VTS 2"); // dial DTMF

        string response = port.ReadTo("\r").Trim(); // read until first newline
        port.Close();

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