How to get phone call confirmation alert dialog Ok Button Action

I have facing one issue regarding phone call confirmation alert dialog. I want known user click "Ok" or "cancel" button.

But, I didn't get action method of "Ok" button. Please help me out. So we known user click on "OK" button.

    NSString *phoneStr = [[NSString alloc] initWithFormat:@"tel:786900**71"]];
     NSURL *phoneURL = [[NSURL alloc] initWithString:phoneStr];
     [[UIApplication sharedApplication] openURL:phoneURL];

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