Allow Only Numbers

<script type="text/javascript">
    //Function to allow only numbers to textbox
    function validate(key) {
        //getting key code of pressed key
        var keycode = (key.which) ? key.which : key.keyCode;
        var phn = document.getElementById('txtSortOrder');
        //comparing pressed keycodes
        if (!(keycode == 8 || keycode == 46) && (keycode < 48 || keycode > 57)) {
            return false;
        }
        else {
            return true;
        }
    }
    </script>

<asp:TextBox ID="txtPhn" runat="server"

onkeypress="return validate(event)"></asp:TextBox>