Thinking about the 100S. Read you need a lot of capacity for the large files. What would I need? Thank you    

Then you would have to know at what frequency the display runs from the mobile phone. Modern mobile phones have significantly higher frequencies than 60Hz and are able to generate variable frequencies, depending on the application. Then there is the problem with polarization. If you can adjust the shutter speed on the camera directly, you should choose a synchronous shutter speed. As an alternative, you can use another mobile phone model. With my Samsung Galaxy S10+ DUOs, I don't have these problems with the Fujifilm X-A1.

Yes.  Excuse the quality as I have had to reduce it from 4K Dscf7475-40.3gp

Do you have an example video?

Anyone have a solution to my problem.  I was shooting 4K today with my XT3 and the shots with a mobile phone screen in it had dark bands crossing the phone screen.  I realise it would be down to frequency but does anyone know of what settings to put into the camera to counteract this issue.  My frame rate was 29.97fps  H.265 (HEVC)/H.264.  My ISO setting was 400 and it was a sunny day. Thanks in advance