I can get the default remind minutes value from user configuration dictionary property ('piRemindDefault'). How do I get the value from EWS if I uncheck the Default reminder checkbox in outlook calendar option. Thanks.

The 'piRemindDefault' properity always return value even I unchecked the 'Default Reminders:' checkbox in calendar option.

The piRemindDefault is just the default reminder period (which is 15 minutes by default) so even when you untick reminders its not going to change the value. You looking at the Calendar Config object to get the reminders option you need to look at the OWA.UserOptions config object in the root of Mailbox eg

            FolderId RootFolderi = new FolderId(WellKnownFolderName.Root, "user@domain.com");
            UserConfiguration usConfig = UserConfiguration.Bind(service, "OWA.UserOptions", RootFolderi, UserConfigurationProperties.All);
            if (usConfig.Dictionary.ContainsKey("enablereminders"))
            {
                Console.WriteLine(usConfig.Dictionary["enablereminders"]);
            }