I wrote a Hello World app, and put the following code in *...ViewController.m*:

```
- (void)viewDidLoad {
    [super viewDidLoad];
    CTTelephonyNetworkInfo *networkInfo = [[CTTelephonyNetworkInfo alloc] init];
    networkInfo.subscriberCellularProviderDidUpdateNotifier = ^(CTCarrier *carrier) {
        [[[UIAlertView alloc] initWithTitle:@"SIM Alert"
                                    message:@"This is an alert."
                                   delegate:nil
                          cancelButtonTitle:@"OK"
                          otherButtonTitles:nil] show];
    };
}

```

Then I unplugged the SIM card but no alert was shown as expected. Could someone kindly tell me what's wrong? Thanks.