Answering or Ending Calls
For the most part, you can both answer and end a phone or FaceTime call with one gesture: the two-finger double tap, also known as the "magic tap". Tap two fingers on the screen, then tap them again quickly. When you are receiving a call, this gesture will answer; when you are in a call, this gesture will end it. When ending a call, though, please note that you must remove the phone from your ear if you're talking on it that way. When an iPhone is held to your ear, its touch screen is deactivated. As you'd expect, the keyboard command that does the same as a magic tap (vo-hyphen by default) will work to answer/end calls.
Sometimes, for reasons unknown, this gesture will not work. In that case, simply find the "answer call" or "Accept" button that will appear on the screen. Alternatively, bring up the phone app (which will launch when a call is answered anyway) and manage the call from there. See the section below about getting back to a call for more on this.
If you want to silence the ringer so you can better hear VoiceOver, press either volume button while the device is ringing. This disables the ringer without declining or answering the call, giving you the chance to hear VoiceOver and use the options available when a call is incoming.
As you'd expect, you can also answer/end a call with a headset, if the headset supports this feature. For instance, you can squeeze the stem of AirPods Pro, double tap either earbud for non-pro AirPods, press the play/pause button on many common headsets, press the same button on wired Apple EarPods, and so on. Some headsets offer more advanced features, such as muting or redialing, but I'll leave those to you, as I can't cover all the headsets in the world.
The final way to manage calls is with an Apple Watch. When a call comes in, your watch will ring, unless you've turned this feature off. You can answer the call on the watch by using the on-screen buttons, and end it the same way. A magic tap doesn't seem to work on watchOS, at least not reliably. When you're in a call on your iPhone, you can open the Phone app on Apple Watch to find the name/number of the person you're talking to, transfer the call to the watch, or end the call. You can't open a number pad on Apple Watch and enter numbers on the call that way, at least not as of the time of this writing.
Bringing Up Call Controls
When you answer a call, you're placed on a screen with controls such as the keypad, a button to end the call, a button to hide the keypad and show other call options, and information on whom you're talking to and how long the call has lasted. However, you are free to do other things with your device while a call is in progress. VoiceOver's speech will sound worse than it normally does, but apart from that, you can use your device as you normally would. Note that dictation (and therefore Siri) isn't available during a call. Also, remember that the position usually used for typing using braille screen input can sometimes make your phone think it's being held to your ear, so using BSI might make a speakerphone call switch to an earpiece call for a bit.
If your screen is locked, pressing the home or lock button once will show the call controls. Tapping or raising the phone to wake the screen, if your device supports this, will do the same thing.
If your screen is unlocked, but you've exited the Phone app, get to the status bar at the top of the screen. Once you land on any status bar item, swipe left or right with one finger until you find an item that details the call in progress. Double tap this to open the call controls. Despite how obvious it sounds, opening the Phone app won't actually get you back to the screen you expect. This is helpful if you need to look up a contact or a recent call while you're talking to someone, but it also means that bringing up the active call is more involved than you might at first think.
Using Touch Tone Menus
Often, you'll have to deal with a menu during a phone call--selecting a department, entering a code, and so on. This is certainly doable on iOS, but it may take some getting used to if you're coming from a phone with a tactile keypad.
The first thing to remember is that, as described above, the iPhone will disable the touch screen when you hold it to your ear. To enter numbers on the keypad, you must lower the phone and wait for the touch screen to come back on. Unless you set it in Settings > General > Accessibility > Call Routing, this will cause your call to switch to speakerphone mode. It has to, or you couldn't hear VoiceOver telling you what key you're pressing, but this is important to keep in mind. For this reason--and because it can be easier to hear speech over the phone call audio--many people prefer to use a headset for this kind of call. You certainly don't have to, of course, but do remember that speakerphone mode will engage once the touch screen comes on.
The second thing to note is that you will need speech on to hear what keys you are touching. Selecting a key is exactly the same as selecting a letter on the iOS keyboard; that is, it follows the typing mode setting you normally use. To enter a number during a call, just find the key and lift your finger (for Touch Typing) or double/split tap (for Standard Typing).
Muting, Joining, and Other Call Management
While in the usual view you get when you make a phone call, you have a numeric keypad taking up much of the screen. Above it is the number (or name, if it's in your contacts) you've called and the duration. Below it are buttons to end the call and hide the keypad. The latter button is how you bring up all the other calling options available. Double tap it while a call is in progress, and you should see your keypad replaced by several buttons--mute, add call, and more. All of these are very self explanatory; the thing I wanted to show you is how to reveal them. Should you need it again, there is a "Show Keypad" button in the bottom right of the screen which will bring your numeric keypad back. If you do nothing with the screen while the keypad is hidden, it will return on its own after a while.