Hi saheem,

Refer below code.

HTML

<input id="txtPhone" type="tel" />
<%--<span id="lblValid" class="hide" style="color: green;">✓ Valid</span>
<span id="lblError" class="hide" style="color: red;">Invalid number</span>--%>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Invalid number" ForeColor="Red" ClientValidationFunction="ValidateCheckBox"></asp:CustomValidator><br />
<asp:Button ID="Button1" runat="server" Text="Submit" />
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/12.1.5/css/intlTelInput.css" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/12.1.5/js/intlTelInput.min.js"></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/12.1.5/js/utils.js"></script>
<style>
    .hide {
        display: none;
    }

    .error {
        display: block;
    }
</style>
<script type="text/javascript">
    $(function () {
        var telInput = $("#txtPhone");
        var code = "+911234567890";
        telInput.val(code);
        telInput.intlTelInput({
            autoHideDialCode: true,
            autoPlaceholder: "ON",
            dropdownContainer: document.body,
            formatOnDisplay: true,
            hiddenInput: "full_number",
            initialCountry: "auto",
            nationalMode: true,
            placeholderNumberType: "MOBILE",
            preferredCountries: ['US'],
            separateDialCode: true
        });
        FormatNumber();
        telInput.keyup(function () {
            FormatNumber();
        });
        telInput.blur(function () {
            if ($.trim(telInput.val())) {
                if (telInput.intlTelInput("isValidNumber")) {
                } else {
                    telInput.addClass("error");
                }
            }
        });
        telInput.keydown(function () {
            telInput.removeClass("error");
        });
        function FormatNumber() {
            var number = telInput.val();
            var classf = $(".selected-flag > div").attr("class");
            var flag = classf.slice(-2);
            var formattedNumber = intlTelInputUtils.formatNumber(number, flag, intlTelInputUtils.numberFormat.INTERNATIONAL);
            telInput.val(formattedNumber.slice(formattedNumber.indexOf(' ') + 1, formattedNumber.length));
        }
    });

    function ValidateCheckBox(sender, args) {
        var telInput = $("#txtPhone");
        if (telInput.intlTelInput("isValidNumber")) {
            args.IsValid = true;
        } else {
            args.IsValid = false;
        }
    }
</script>