title: Send USSD code to modem in C# and ERROR in return always
body: <p>I am sending USSD code on modem through serial port. But always it is giving ERROR in response.</p>
  
  <p>AT commands I am sending are:
  in sequence:</p>
  
  <pre><code>serialPort.Write("AT+CMGF=0" + "\r\n");
  serialPort.Write("AT+CUSD=1,\"*135#\"" + "\r\n");
  </code></pre>
  
  <p>when I am configuring message format in first AT command, it is giving 'OK' response. But on sending USSD code, response is always 'ERROR'. Why is it so? </p>