iOS 实现拨打电话功能

Question

我需要实现拨打电话，但是使用网上的代码点击后毫无反应，为什么？
NSMutableString * str=[[NSMutableString alloc] initWithFormat:@"tel:%@",@"137********"];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];

Answer 1

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"400-820-8820" message:nil preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *cancelAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {

    }];
    UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) { NSMutableString  * str = [[NSMutableString alloc] initWithFormat:@"tel:%@",@"4400-820-8820"];
        [[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];
    }];

    [alertController addAction:cancelAction];
    [alertController addAction:okAction];
    [self presentViewController:alertController animated:YES completion:nil];