Validate Indian Mobile Number in ASP.Net TextBox

I have made sample for you.

HTML Code

<asp:TextBox ID="txtMobileNo" runat="server"></asp:TextBox>
        <asp:Button ID="btnSave" runat="server" Text="Submit" ValidationGroup="ValidateMobile" />
        <asp:CustomValidator ErrorMessage="Not a valid mobile number" ClientValidationFunction="ValidateMobileNumber"
            runat="server" ValidationGroup="ValidateMobile" />

Script

<script type="text/javascript">
        function ValidateMobileNumber(sender, args) {
            args.IsValid = false;
            var mobileNumber = document.getElementById('txtMobileNo').value;
            if ((mobileNumber.substring(0, 1) == 9 || mobileNumber.substring(0, 1) == 8 || mobileNumber.substring(0, 1) == 7)&&(mobileNumber.length==10)) {
                args.IsValid = true;
            }
        }
    </script>