Cellphones may record video footage at 30 frames per second (FPS), using a variable frame rate (VFR). To use these videos with some other footage recorded at 25 FPS (and edit at 25 FPS) the phone's video has to be re-encoded at 25 FPS. A basic conversion will only drop frames, and the result won't be smooth, especially when panning. A better conversion requires interpolation, like that of the filter minterpolate of FFmpeg. To convert using FFmpeg use the command:
ffmpeg -i IMG_0001.MOV -filter:v "minterpolate='fps=25'" -c:a copy IMG_0001_25fps.mp4
This will be slow, but the end result is much better.