You can use a javascript library like Mask.
For a phone number you can try a mask like this new Mask("### ### ####");
So for a complete example :
<html>
<head>
<script src="./masks.js"></script>
<script>
function init(){
document.frmExample.reset();
oPhoneMask = new Mask("### ### ####");
oPhoneMask.attach(document.frmExample.phone_number);
}
</script>
</head>
<body onload="init();">
<form name="frmExample" method="get" onsubmit="return false;">
<b> Phone Number:</b><br />
<input type="text" name="phone_number" value="" /><br />
Generic: ### ### ####
</form>
</body>
</html>