First of all, iPad is not a cell phone, so the native phone app on iPhone would be of no value to an iPad. Even if you purchased a cellular iPad, it is for data only as iPad does not have a cellular radio like a cell phone.
Since you admit this is your first Apple device, you won't be able to make or receive phone calls on the iPad. If you owned an iPhone, you would be able to via Apple Continuity Services, but that won't help you if you don't have an iPhone. There are 3rd party apps you can use, such as Skype.
Apple just doesn't include a calculator app natively on iPad, but there are loads of Calculator apps, many which are free, you can download from the App Store.