title: SMS send error from C# application using GSM modem
body: <p>I develop a C# application that has (text mode) SMS communication capabilities. Usually it works fine, but sometimes SMS sending goes wrong. Here is what SOMETIMES happens:</p>
  
  <p>AT+CMGS="+3630xxxxxxx"</p>
  
  <p>> text CTRL+Z</p>
  
  <p>Normally it should answer this:
  +CMGS: SMS ID\r\n OK\r\n</p>
  
  <p>Sometimes, it does not answer anything after sending CTRL+Z even after 30 seconds, the CMGS:  answer does not come from the modem, sometimes the answer is the SMS text itself. After I try to send another SMS, it answers +CMS Error: 515. It means, that modem is busy. I think it is busy because there is an SMS in the outbox. Note, that other AT commands still work perfectly.</p>
  
  <p>So, what to do in this situation? ...when +CMGS: SMS ID\r\n OK\r\n does not come when sending SMS?</p>
  
  <p>I use .net framework2 and its serial communication features. Modem is a Simcom 300 d.</p>
  
  <p>Pls. help me, thanks in advance!</p>