I'll go ahead and chuck my opinion in the mix here.
First off, nicely done. It's very difficult to subject work to the public, especially one as learned as this group. Looks like everybody has been very nice so far :)
I really like the bell sound. It would be nice for it to feel boxier when it's just darkness - you don't get a sense for the space until the door is opened. The high frequencies ring like it is in a large room.
The door is really nice, but could probably come down quite a bit. Would have liked to hear some sort of handle grab with a latch open. The door is tightly closed, so it seems like it would have been latched in some way. Nickpicking a little on that point, I guess....
Needs some work on perspective, even if it is just tweaking the level of the phone up and down as you change perspective. On the wide angle of her answering the phone, her voice has a nice metallic reverb to it - can you make the bell sound the same way?
As for ambiance - I've always struggled with world building on my own. I like the tension that it brings, but would like to maybe get a few more audio clues as to where she is - active factory, a ship, underground, in a dream world, etc. This is a lot to pack in to 30 seconds.. a longer clip would help in that aspect, but you can start introducing some elements right away.
Again, excellent job and keep up the good work! Look forward to seeing some of your submissions in the sound design challenge.