I want to restrict my app only for iPhones. I don't want my app to be available for iPads.
The app UI is suitable for the iPhones, whereas it's not optimized for iPads yet. Since, the UI optimization for iPads will require substantial time and effort, we'll do that later on.
Is there any way to solve this problem?
AFAIK you cannot prevent installing on iPad, at least in compatibility mode. Then the set up will remain unchanged, just a smaller window.
You could test when opening the app if it is iPad. And act accordingly. But take care, depending on what you do not to be rejected during appstore review.
However, if setup is optimised for iPhone, have you checked it works on all iPhone formats ? Both in Portrait and landscape ?
Can we release an app only for ipad and not for iphone for now and later release a new build to iphone?