What's CTSubscriber (and how to use it) on iOS 7?

Question

On iOS 7, CTSubscriber was added to the CoreTelephony framework. There is no documentation available, only its header file:

/*
 * CTSubscriberTokenRefreshed
 *
 * Description:
 *     The name of the NSNotification sent when the carrier token is available.
 */
CORETELEPHONY_EXTERN NSString * const CTSubscriberTokenRefreshed  __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_7_0);

CORETELEPHONY_CLASS_AVAILABLE(7_0)
@interface CTSubscriber : NSObject

/*
 * carrierToken
 *
 * Description:
 *     A data blob containing authorization information about the subscriber.
 *
 *     May return nil if no token is available.
 */
@property (nonatomic, readonly, retain) NSData* carrierToken  __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_7_0);

@end

Also, on What's new on iOS 7, this is mentioned:

The Core Telephony framework (CoreTelephony.framework) lets you get information about the type of radio technology in use by the device. Apps developed in conjunction with a carrier can also authenticate against a particular subscriber for that carrier.

I think that CTSubscriber is related to the bold part of the text. However, I haven't found anything related on how this happens.

I have tried to use the following code (added to application:didFinishLaunchingWithOptions:) to experiment with this API, but the notification is never fired and carrierToken returns nil:

CTSubscriber *subscriber =  [CTSubscriberInfo subscriber];
NSLog(@"%@", subscriber.carrierToken);

[[NSNotificationCenter defaultCenter] addObserverForName:CTSubscriberTokenRefreshed object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
    NSLog(@"==========");
    NSLog(@"%@", note);
    NSLog(@"%@", subscriber.carrierToken);
}];

So, I have the following questions:

• What exactly ("authorization information") does carrierToken return and how to make it not nil?
• How does Apple know if your app is "developed in conjunction with a carrier"?
• Is this how Evernote is giving 1 year of premium account to Telefonica users (http://blog.evernote.com/blog/2013/08/13/evernote-and-telefonica-announce-global-partnership/)? (Probably not, since the information they need can be obtained on CTCarrier)

Answer 1

I asked the same question in the developer forums and got this reply :

You should escalate this via the carrier you're working with, who can in turn escalate it to their contact at Apple.

Link to the thread: https://devforums.apple.com/message/934226#934226