Hi there!
My partner recently acquired a Samsung Galaxy S5 phone, and we've discovered that, regardless of format (MP4 or 3GP), when a video is sent as an e-mail attachment, it absolutely refuses to play, no matter if played from the e-mail, or played as a download on the PC itself. Nor will it play on Windows Media Player, it seems, so it's clearly not just a problem with VLC.
My guess is, the phone being so new, it's using some new kind of compression for the HD video that VLC has no codec for? But I'd value insight and input from anyone who might be able to offer a way to bypass this. I've tried convert/save, no joy, and even using another program to change the format, but that program couldn't "find" the video either!
Anyway with this phone being so new, I figured no-one might know about this issue yet, so thought my post might prove useful. Thanks for listening.