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.
Siri may have problems helping you make phone calls if your Contacts aren't set up as intended. You may have transferred contacts from an old phone which had a rudimentary contacts system which forced you to be a bit creative in the way you set up contacts. You may have added extra information inside fields like first name or last name. Contacts on your iPhone and iPad have lots of fields for all sorts of information and it's worthwhile considering cleaning up your contacts if you initially have difficulty using Siri for phone calls. Please take a look at my advice on setting up contacts.
Siri can be instructed to make phone calls with commands like:
"call John Smith" or
"call 0800 0480754"
"call Sainsburys" - you don't need to have most businesses in your contacts since Siri will look up the number for nearby Sainsburys stores and ask you which you want.
You'll find more about phone calls and Siri in Siri Essentials.
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 also applies to incoming FaceTime and WhatsApp calls.
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 anywhere on the screen. You may then hear a tone to indicate that the call is ended. This technique also applies to FaceTime and WhatsApp calls. Be aware that if the person at the other end of the call ends the call before you then your double tap with two fingers may resume playing media you were playing on your phone. No problem, just double tap again with two fingers to pause the media. The two finger double tap gesture is called the magic tap because it does different jobs in different contexts.
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.
For full access to all the phone features on your iPhone use the Phone app, which is probably in the dock, the bottom row of icons on your iPhone screen. Once you have opened the Phone app 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.
Favourites, recents, contacts and keypad all allow you to make calls. Favourites is a list of contacts you have nominated as favourites using the add button on the Favourites screen. 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 section index is probably best for a long list of contacts. Swipe right from the top of the screen until you hear "section index, adjustable" and then swipe down with one finger until you reach the letter you want in your contact list. Next swipe right until you reach the desired contact or left if you have already passed the desired contact. 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 / Apps / 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.