Question

W:Hello. May I speak to Mr. Johnson?

M: Speaking.
1. Who's calling, please?
--------------------------
2. Who's talking, please?
3. Who's speaking, please?

(Can we use #2 and #3 instead of #1 in telephone conversation?)

Answer 1

In America, #1 is the preferred, most common. The others are okay, but not used here. Perhaps in other places, they are more commonly used.

Answer 2

I've heard people use 1 and 3, but I've never heard anyone use 2.

As Reed wrote, 1 is the most commonly used in the US.