Validate Phone Number length based on country using Custom Validator in ASP.Net

You can use the custome validation for this purpose:

<form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="ddlCountry" runat="server">
            <asp:ListItem Text="India" Value="0" />
            <asp:ListItem Text="Nepal" Value="1" />
            <asp:ListItem Text="Pakistan" Value="2" />
        </asp:DropDownList>
        <asp:TextBox ID="txtPhoneNumber" runat="server" />
        <asp:CustomValidator ID="cvPhoneNumber" runat="server" ControlToValidate="txtPhoneNumber"
            ErrorMessage="Invalid PhoneNumber" ClientValidationFunction="CheckPhoneNumber"></asp:CustomValidator>
        <asp:Button Text="Save" runat="server" />
        <script src="Scripts/jQuery_1.8.3.js" type="text/javascript"></script>
        <script type="text/javascript">
            function CheckPhoneNumber(sender, args) {                
                var country = $('[id*=ddlCountry]').val();

                switch (country) {
                    case "0":
                        if ($('[id*=txtPhoneNumber]').val().length != 10) {
                            args.IsValid = false;
                        }
                        break;
                    case "1":
                        if ($('[id*=txtPhoneNumber]').val().length != 7) {
                            args.IsValid = false;
                        }
                        break;
                    case "2":
                        if ($('[id*=txtPhoneNumber]').val().length != 8) {
                            args.IsValid = false;
                        }
                        break;

                    default:
                        args.IsValid = false;
                }
            }
        </script>
    </div>
    </form>

Thank You.