With an iPhone you can still practise framing/composition, shooting in a way that helps creating a good edit. The best camera to use now, is the one you have access to.
And if you really want to be a filmmaker, you don't want to stop yourself from starting by searching for a camera while you have one in your pocket.
You can start NOW.
Shoot a little scene, edit and learn from it.
I know this sounds like being a smartass, but I've seen several people who knew 'for fact' that they wanted to be a filmmaker, but in the end their gear was just collecting dust, because the reality of filmmaking was a lot less glamourous than they imagined.
On the other side: my first project was shot on a crappy webcam in 2000. The resolution was extremely low, the image quality was bad, it was black and white and the computer could just handle 14 frames per second. I couldn't afford a camera and it didn't stop me.
And it was FUN

After that I made a short animation with pencil and paper and a scanner.
Only after I knew this is truly my passion I invested in gear.
So, since you made up your mind:
what kind of shorts would you like to make?