Whilst there are modules around including the method mentioned in a previous post , I think you could easily do this with just the text/html module and posting to and SMS gateway something like (this is just an idea):

<div id="container">
<h1>Sending SMS with DNN</h1> 
     <ul>
      <li>
       <label for="phoneNumber">Phone Number</label>
       <input type="text" name="phoneNumber" id="phoneNumber" placeholder="3855555555" /></li>
      <li>
      <label for="carrier">Carrier</label>
       <input type="text" name="carrier" id="carrier" />
      </li>
      <li>
       <label for="smsMessage">Message</label>
       <textarea name="smsMessage" id="smsMessage" cols="45" rows="15"></textarea>
      </li>
     <li><input type="submit" name="sendMessage" onclick="PostThisForm()" id="sendMessage" value="Send Message" /></li>
    </ul>
</div>



You will need to override the DNN form submit function.  Some javascript

function PostThisForm(){
/* How the message is formulated and what the SMS gateway requires will be different for each vendor */
document.forms[0].action = "url of sms gateway + message and recipient"; 

document.forms[0].submit(); 
return true;
} 





Some css

#container {
 width: 600px;
 background: #fff;
 color: #555;
 border: 3px solid #ccc;
 -webkit-border-radius: 10px;
 -moz-border-radius: 10px;
 -ms-border-radius: 10px;
 border-radius: 10px;
 border-top: 3px solid #ddd;
 padding: 1em 2em;
 margin: 0 auto;
 -webkit-box-shadow: 3px 7px 5px #000;
 -moz-box-shadow: 3px 7px 5px #000;
 -ms-box-shadow: 3px 7px 5px #000;
 box-shadow: 3px 7px 5px #000;
}


ul {
 list-style: none;
 padding: 0;
}


ul > li {
 padding: 0.12em 1em
}


label {
 display: block;
 float: left;
 width: 130px;
}


input, textarea {
 font-family: Georgia, Serif;
}


so forth and so on.


Hope this helps


Marcel