Sorry for the long wait. OpenToonz shouldn't have a problem importing mp4 files.
You've found the issue yourself: the video is 12fps on your phone. But when you bring it onto your computer it becomes 30fps.
This has nothing to do with OpenToonz. Whatever method you're using, you'll have to look up why it's causing such an issue.
Cheers.
P.S.: Mate, you're still privately replying. Come to the Google Groups website and click the "Reply All" button for this conversation.