JSON url response null

I write an application for iphone in objective-c and wanna get data from json. but I get null from url, but url is correct and when I pass url to browser I see the json data. this is my IBAction method:

- (IBAction)checkMobileNumber:(id)sender {
    NSString *prefix = self.prefixTextField.text;
    NSString *number = self.numberTextField.text;
    NSString *url =[NSString stringWithFormat:@"http://ift.tt/15Q9nsW", prefix, number];
    NSURL *jsonURL = [NSURL URLWithString:url];
    @try {
        [[[NSURLSession sharedSession] dataTaskWithURL:jsonURL completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            NSString* rawJSON = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            NSError *err;
            self.checkMobile = [[CheckMobileProviderModel alloc] initWithString:rawJSON error:nil];
            if (err) {
                NSLog(@"Unable to initialize PublicPhotosModel, %@", err.localizedDescription);
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                NSLog(@"%@", self.checkMobile.response);
                //NSLog(@"%@", self.checkMobile.fault[@"faultString"]);
            });
        }] resume];
    }
    @catch (NSException * e) {
        NSLog(@"Exception: %@", e);
    }
}

what is wrong here? any help?