IOS Jailbreak How do intercept SMS / Text Messages

Although the poster already accepted rajagp's answer, I'm pretty sure it doesn't do what the question actually asked, on iOS 5. For iOS 5, I'm no longer seeing the message content anymore, although I do get notified that there is a new message.

So, what I did is take rajagp's notification handler for kCTMessageReceivedNotification, and inside it, use the code posted here to actually get the content of the text message, from the SMS database.

This code snippet should intercept SMS messages- You can extend it for other kinds of notifications. Will work on iOS 5.0.1 as well. Does not work with iMessages though. Link with CoreTelephony framework (there are bunch of private headers there which you'd can class-dump)

#include <dlfcn.h>

#define CORETELPATH "/System/Library/PrivateFrameworks/CoreTelephony.framework/CoreTelephony"
id(*CTTelephonyCenterGetDefault)();

void (*CTTelephonyCenterAddObserver) (id,id,CFNotificationCallback,NSString*,void*,int);


static void telephonyEventCallback(CFNotificationCenterRef center, void *observer, CFStringRef name, const void *object, CFDictionaryRef userInfo)
{
    NSString *notifyname=(NSString *)name;
    if ([notifyname isEqualToString:@"kCTMessageReceivedNotification"])//received SMS
    {
        NSLog(@" SMS Notification Received :kCTMessageReceivedNotification");
        // Do blocking here. 
    }
}

-(void) registerCallback {

 void *handle = dlopen(CORETELPATH, RTLD_LAZY);
    CTTelephonyCenterGetDefault = dlsym(handle, "CTTelephonyCenterGetDefault");
    CTTelephonyCenterAddObserver = dlsym(handle,"CTTelephonyCenterAddObserver");
    dlclose(handle);
    id ct = CTTelephonyCenterGetDefault();

    CTTelephonyCenterAddObserver(
                                 ct, 
                                 NULL, 
                                 telephonyEventCallback,
                                 NULL,
                                 NULL,
                                 CFNotificationSuspensionBehaviorDeliverImmediately);
}