<script>
function checkForm() {
//check for phone number here...
phoneNo=document.getElementById("phone").value
if (phoneNo.match(/^\d\d\d\-\d\d\d-\d\d\d\d$/)) return true;
else {
alert("Please check your phone number")
return false
}
}
</script>
<form method="get" action="url" onsubmit="return checkForm()">
<input type="text" id="phone">
...
<input type="submit">
</form>