I have some developers creating an iPhone app, often the app will not work on one type of iPhone even though it worked on another one using the same version of iOS. Therefore, I am looking for a service where I can test the app natively on all the iPhone versions running various versions of iOS. I would like to be able to interact with the iPhones myself, so that I know that a specific bug has actually been fixed before, pushing to App Store and waiting 9 days for the review before I can hear the sad news from customers. Googling got me nowhere.
Do such services exist?