一，效果图。

二，代码。

RootViewController.m

- (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];
}