You would have seen in many recharge websites,
After entering your mobile number, operator will be chosen automatically. Here you can find how they have implemented that with sample snippet,
[html]
<html>
<head>
<script type="text/javascript">
function Operator(input){
var element = document.getElementById(‘oprCode’);
if(input == 9943){
element.value = "VF";
}else if(input == 9962){
element.value = "AT";
}else if(input == 7200) {
element.value = "TD";
}else if(input == 8124) {
element.value = "ID";
}
}
</script>
</head>
<body>
Enter Your Mobile Number <input type="text" onkeyup="Operator(this.value);">
<br/>
<br/>
Operator is : <select id="oprCode">
<option value="default">——</option>
<option value="ID">Idea</option>
<option value="TD">Tata Docomo</option>
<option value="AT">Airtel</option>
<option value="VF">vodafone</option>
</select>
</body>
</html>
[/html]
Output:
