Suppose, one of my friends is waiting for me on street. I'm going to ground floor using stairs of our building. At that moment if my friend phones me, what should I say to mean that I'm on the way to street? Is "I'm coming down the stairs" okay? I just want to know, how does a native speaker respond to the question 'Where are you?' in the described situation?
"I'm on my way" or "I'm almost there"
Either of these would be a typical, native-speaker, response to "where are you?" in the OP's context.
Your friend is in front of your building, therefore your friend has a very good idea of the farthest away you can possibly be, as well as a decent idea of all the potential locations in between. So there is no need to give an absolute location, just address the 'question behind the question,' which is: "when will you be here?"
They really don't care where you are, they want to know how long they'll have to wait for you.
Echoing the comments, any of those are fine. Specifying the method by which you are descending (e.g. I'm taking the elevator down OR I'm taking the stairs down) may sound a little strange unless it is critical for your friend to know where to meet you (i.e. if the stairs lead to a different door to exit than the elevator and you want your friend to know that).
For comparison, if my boss were to ask me where I am in the morning if I'm running late I'd say "I'm leaving my house" rather than "I'm leaving my house through the front door." Just unnecessary information.
There are many potential responses, almost all informal. Here are a few
Parenthesis are words sometimes added, but not always
- I'm on my way
- I'm coming down (now)
- I'm on my way (down)
- I'm nearly there
- I won't be a minute
- I'll be out in a moment/minute/second
Generally we won't specify the mode of transport unless it's particularly relevant. Note that in the above, we don't say "Down the stairs", and in most cases don't even specify "down".
We'd only usually specify the mode of transport when it matters: for example at my office there are lifts (elevators) at one end of the corridor, and stairs at the other: so if someone is meeting me downstairs we'll specify which we're using.