Prova questo:
codice:
<html>
<head>
	<title>Untitled</title>
<script language="JavaScript">
<!--
function checkThis() {
	var myField = document.dati.telefono;
	if (isNaN(myField.value)) {
		myField.value=myField.value.replace(/[^0-9]/gi,"");
		myField.focus();
		alert('dato non numerico!');
	}
}

//-->
</script>
</head>
<body>

<form name="dati">
<input type="text" name="telefono" onKeyUp="checkThis();">
</form>

</body>
</html>