Hi
The Phone app cannot be deleted or hidden.
You may have moved it from the Dock to a different Home screen and/or into a folder.
To search for the Phone app, when viewing the Home screen on your iPhone:
- Swipe down on the centre of the screen > in the Search box at the top of the screen, type "phone" > the app icon will be shown (you can open it from there) along with any folder name, shown to the right hand side of the icon.
Having located it, you may then wish to drag it back into the Dock: touch and hold your finger on the app icon until it starts jiggling, then drag it to the Dock. Press the Home button once when done.