I know this is an old post, but I'll give a different answer.
esqew approach works well but it goes straight to the phone app and it doesn't give you a prompt to confirm the number with a call button.
Try this approach and it gives a prompt option.
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"telprompt://1028464023"]];
The only thing that changes is: telprompt on URLWithString. Give it a try.