IOS如何替换电话号码中间4位为"-"符号

- (void)viewDidLoad
{
 [super viewDidLoad];
 // Do any additional setup after loading the view.
 
 //电话号码
 UILabel *telLabel = [[UILabel alloc] initWithFrame:CGRectMake(10, 100, 200, 20)];
 telLabel.textColor = [UIColor grayColor];
 telLabel.text = @"13793333281";
 //字符串的截取
 NSString *string = [telLabel.text substringWithRange:NSMakeRange(4,4)];
 //字符串的替换
 telLabel.text = [telLabel.text stringByReplacingOccurrencesOfString:string withString:@"----"];
 [self.view addSubview:telLabel];
}