Stop. Seriously, stop it.
Not only are you creating traffic, you yourself have become an obstacle. People have to step around you so that they can walk down the stairs. That is very inconvenient. Why can’t you check your phone before you walk halfway down to the subway? How about next time, you take your call elsewhere and not on the middle of the steps during rush hour.
The next time this happens, I will make sure you know how irritated I am. Beware.