The Phone app comes built-in on every iPhone and can not be deleted. Whenever you want to make a phone call or answer one, it's handled via the Phone app. You can access contact cards within the Phone app, create a Favorites list that contains the people you communicate with the most, and even respond with a text message automatically when you aren't able to answer a call. As for voicemail, the iPhone's Phone app features a visual voicemail app that lets you listen to your messages on demand whenever you'd like.
Once you have active cellular service on your iPhone, the Phone app will be your central hub for communicating with people, aside from the Messages app. Once you have information for all your contacts, you can use the Phone app to initiate a phone call, text message, FaceTime audio or video call, and an email. While the iPhone also comes with a Contacts app, it's also built-in to the Contacts tab of the Phone app for convenience.
For times when you can't answer your iPhone, the Phone app gives you many options for managing incoming calls. You can simply decline the call, send an automatic text message saying you can't talk, or set a reminder so you remember to call that person back when you're next available. If they leave a voicemail for you, just tap on the Voicemail tab in the Phone app in oder to access Visual Voicemail. This is also where you will want to go to set up a custom greeting for callers to hear when your voicemail picks up. Other main sections of the Phone app consist of a recent calls section and a keypad for when you need to dial a number that isn't in your address book already.