break down vs break?

Hi everybody, I have a question about differences between “break down” and “break”.
Could anybody please tell me?
I read the book “really learn 100 phrase verbs” and got stuck in practise section of phrase verb “break down”.
here it is :
break or break down? choose the best alternative :
“My new mobile phone’s broken/broken down.” - “Did you drop it again”.
the right answer is “broken” but I can’t understand why.
Please explain it for me.
Thank you!

As I learned

"break down " means the to break to subunits, such as break the stick to equal parts

The other one actually I don’t know

well, thank you, Hyder12.
anyone else? please help!

“Break down” doesn’t only mean to break into subunits or small pieces. It often also means to stop functioning, as when a car breaks down. “Break down” is usually used when talking about people or big machines.

When small things stop functioning, like a phone or a radio, we say that they’re broken.

I understood.
thanks for your help, Jamie (K).