Can you describe what is actually "breaking" or attach a screenshot from the phone (press home button and on off button at the same time). I have experience in mobile email optimization and I'm sure I'll be able to answer your question if i could understand what is actually getting messed up. I'll try to send your code form outlook to my phone, but I have an iPhone running ios7, it may look just fine.
EDIT:
Without actually testing your code I see that your media query was incorrect.
You have:
@media only screen and (max-device-width: 480px)
Try:
@media only screen and (max-width: 480px)
I'm not sure if this is all you needed, but it worked in the browser when I resized the window.
EDIT:
Here's how it looked on my phone sent as a signature from Mac Outlook 2011 to a gmail account using Apple Mail. The signature was added not with code, but by copying the the browser page in Chrome and pasting into the signature editor.
http://dolf.co/FILES/screenshots/IMG_0702.PNG