how to validate phone and mobile number in javascript

function Validate()
        {
            var x = document.form1.txtPhone.value;
            var y = document.form1.txtMobile.value;
           if(isNaN(x)||x.indexOf(" ")!=-1)
           {
              alert("Enter numeric value")
              return false; 
           }
           if (x.length>8)
           {
                alert("enter 8 characters");
                return false;
           }
           if (x.charAt(0)!="2")
           {
                alert("it should start with 2 ");
                return false
           }
 
           if(isNaN(y)||y.indexOf(" ")!=-1)
           {
              alert("Enter numeric value")
              return false; 
           }
           if (y.length>10)
           {
                alert("enter 10 characters");
                return false;
           }
           if (y.charAt(0)!="9")
           {
                alert("it should start with 9 ");
                return false
           }
        }
 

<script type="text/javascript">
    function onlyNumbers(event) {
        var e = event || evt; // for trans-browser compatibility
        var charCode = e.which || e.keyCode;
        if (charCode > 31 && (charCode < 48 || charCode > 57))
            return false;
 
        return true;
    }
</script>