Unless you’re going for a certain look, you should always shoot videos on your cell phone in a landscape position. Meaning, turn your phone sideways! This will prevent those black bars from showing up on either side of the video.
If you’re going the DIY route for your business’ content, this will make the videos that much more professional looking.
If you’re using an iPhone, you can even adjust exposure (the little sun looking slider that appears when you tap your screen). Always make sure the subject of the video is properly exposed and this is a great way to ensure that.