The phone function of your iPhone is just one of the many apps on your iPhone. It just happens to be the app that lets you make and receive phone calls. You may not need to use the app explicitly very often, though, because Siri lets you make phone calls with spoken commands and accepting incoming calls and ending calls needs only a simple VoiceOver gesture, a two finger double tap.
You can make calls using Siri commands like:
"call John Smith" or
"call 0800 0480754"
If you want to be able to make a call to someone by their name then their name and telephone number needs to be in your Contacts.
When your iPhone rings, VoiceOver will announce the caller and you can accept the call with a two finger double tap anywhere on the screen. If it's not convenient to take the call, it can be dismissed with two short presses of the side button. This will probably send the call to your voicemail service if you have one enabled.
When you make a call and the call is established, a numeric keypad will be displayed on the screen so that you can enter numbers to respond to automated call-handling systems. Slide your finger or swipe to find the keypad numbers and double tap as usual to type the number.
When you want to end a phone call, use the same gesture as you would use to answer an incoming call; double tap with two fingers. You may then hear a tone to indicate that the call is ended.
More options are available in the phone app, which is probably in the bottom, dock row of icons on your screen.You will find 5 tabs at the bottom of the phone app screen. Reeding from left to right these are:
Favourites, Recents, Contacts, Keypad and Voicemail.
If you are unable to use Siri, then favourites, recents, contacts and keypad all allow you to make calls. Favourites is a list of contacts you have nominated as favourites. Recents is a list of recently called, answered and missed calls. You can select items from these lists and double tap to make a call. Keypad is a conventional telephone keypad. Contacts is your full set of contacts. There are several ways of finding a contact. The table index is probably best for a long list of contacts. Swipe right from the top of the screen until you hear "table index, adjustable" and then swipe down with one finger until you reach the letter you want in your contact list. Then swipe right until you reach the desired contact and double tap to open the contact page. Swipe right until you hear "call, button" and double tap to make the call. If you have more than one phone number for that contact you will be presented with a choice to swipe through before the final double tap. Contacts can be organised either by first or last name, depending on a setting in Settings/Contacts.
The way in which the Voicemail tab operates will depend on your mobile service provider and you will need to explore the screen to determine the options available. Some mobile services will offer a "call voicemail, button" in the top half of the screen as well as a button to call network provider VoiceMail. With my network provider, these two buttons have the same effect. I'm unsure how they might differ. Some network providers will offer Apple's visual VoiceMail which will provide you with a list of VoiceMail messages on screen and allow you to listen to them in any order you choose.