It does work almost all of the time. I'm not familiar with iOS quirks, but apparently from the things I've heard, taking a call during the purchase interrupts the purchase, as do apparently other things. Believe me, I have been forcefully making the case with Z that we need to find out what is causing the client to fail to complete the operation. Having said that, I have not heard of a case where an iTunes receipt was not issued. That would be a new one.