title: Error: Invalid number format while sending SMS with SIM800L
body: <p>I'm using a SIM800L GSM module connected over USB-Serial to my computer.
  When I try to send a SMS I got an error:</p>
  
  <pre><code>AT+CMGS="+4915xxxxxxxxx"
  &gt; Test (Ctrl+Z)
  +CMS ERROR: Invalid number format (incomplete number)
  </code></pre>
  
  <p>I’m obviously connected to the network because pin is entered +CREG an +COPS seems good.</p>
  
  <pre><code>AT+CPIN?
  +CPIN: READY
  
  OK
  AT+CREG?
  +CREG: 0,1
  
  OK
  AT+COPS?
  +COPS: 0,0,"O2 (Germany)"
  
  OK
  </code></pre>
  
  <p>As well I can receive SMS and see incoming calls.
  I set the module in sms text mode and use the coding GSM</p>
  
  <pre><code>AT+CMGF?
  +CMGF: 1
  
  OK
  AT+CSCS?
  +CSCS: "GSM"
  
  OK
  </code></pre>
  
  <p>I try different number formats like "015xxxxxxxxx" or "004915xxxxxxxxx". Also I try different values for the optional  parameter from +CMGS</p>
  
  <p>Could anybody advise me what to do?</p>