I've deleted everything and my 15 GB iPhone still has 12 GB of "other" data which continues to grow. How do I delete it? I got this phone a week ago.

I recieved this phone last week, and it was working fine, until a few days ago when I noticed the "other" bar growing, and soon enough, I start getting warnings that my storage is low. I've deleted nearly all of my apps and I've removed all of my music (combined which took up less than 4 GB) and I'm still getting storage warnings. What do I do? Diagnostics didn't go through when I tried, and I wasn't able to reach any support staff because the Apple website page kept failing.

Please help. I'm at the end of my rope.