Here's why:
* "Who am I speaking with?" is the standard, grammatically correct way to ask who you are talking to.
* "Talking to" is less formal and might sound slightly awkward in this context. While you could technically use it, "speaking with" is generally preferred for a more professional and polite tone.
Here's an example of how you might use both in conversation:
* Formal: "Excuse me, who am I speaking with?"
* Informal: "Hey, who am I talking to?"