I delete phone app. How to get it back?

The iPad (the forum you posted in) doesn't have a Phone app. If the question is about an iPhone, the Phone app is not an app that can be deleted. See: iTunes Store - Contact Us - Apple Support

If an iPhone, have you tried searching for it by sliding down from the middle of the home screen and using search?

There isn't a 'phone app' on iPads, that is only on iPhones - your sim card gives you cellular data access, it doesn't turn the iPad into an iPhone