Eight Tips To Shoot Video that don't SUCK with a Phone.
#1 Hold the phone in landscape mode
#2 Keep the phone steady.
#3 Hold the phone in landscape mode
#4 Don't hold the phone in portrait mode
#5 Don't make jerky movements with the phone.
#6 Start your video with an Establishing shot. 
    (shows the subject and the environment around it)
#7 To do #6, hold the phone in landscape mode.
#8 Don't hold the phone in portrait mode!

References:

How to properly hold your phone when you record videos
STOP RECORDING VERTICAL VIDEOS!
people who record videos vertically
Vertical Video is bad
iPhone: Horizontal vs Vertical

Still think it's ok to shoot video in vertical mode? Well, take a look at the two short clips I made side-by-side. See if you don't agree that the second clip is much easier on the eyes.

I rest my case!

Now, if you happen to be using your cell phone camera for a Zoom meeting, or Facebook Live, here are four additional tips from Al Roker [yes, that Al Roker from NBC News]
1. Shoot from above - You don't look good when the camera is looking up your nose. The phone shouldn't be in your lap. It should be about even with your forehead or higher.
2. Come out of the shadows - Make sure the light illuminates your face.
3. Keep clutter out of the background - Flowers or bookshelves are great.
4. Don't face the camera directly - Turn your head slightly to one side, then turn your eyes directly at the camera.