As iPhone models have advanced, Apple has been changing the process for resetting (rebooting) iPhones. Up until the 6s model it was a matter of pressing and holding the power button and the home button, at the same time, until the phone screen turned off and the Apple logo appeared.
With the iPhone 7 you would press and hold the Volume Down button and the Power button and wait for the screen to turn off and the Apple logo to appear.
Beginning with the iPhone8, the process has changed to a sequence. Press and release Volume Up then Volume Down then press and hold the Power button until the Apple logo appears.
Knowing how things have changed is important because now, should you resort to the old "press and hold" technique, the results are much different. I learned this when I was attempting to reset my new phone. I pressed and held a volume button and the power button and as I waited I noticed an emergency call (911) was being initiated. Fortunately the screen shows a countdown and you can release the buttons without calling 911.
During the same "how do I reset my phone" episode, I pressed the power button rapidly several times (at least five times). I learned this also (by default, although it can be disabled) starts an emergency call. Since the call was in progress I stayed on until the call was answered and I explained the call was an accident. (I didn't get in trouble.) Should you accidentally call 911, don't hang up. Emergency service dispatchers must treat every call as an emergency. If you hang up, it takes time and resources as they have to call you back and if they can't reach you may send out the police to check on you.  
By the way. If you initiate the Emergency SOS feature TouchID or FaceID is disabled and you will need to enter your passcode to unlock your phone. This feature keeps a malicious person from access to your phone by pointing the phone at your face or placing your finger on the reader.
This is an example of "version freeze." Many (most) users may never realize software and device enhancements as we continue to do things out of habit. Usually based on the version we first learned.