Why do we say "It's me (or our name)" when we answer the phone? Why do we use "it" to answer the phone? [duplicate]

It's Paula Poundstone seems to me to simply be the answer to an
(unspoken but presupposed) question

Q: Who is it?
A: It's Paula.

A question like Who is this person? is taken as a given in any formal introduction.
And this is the introduction of a number of speakers on stage before a performance.

There are special conventions for this context, as there are for telephone conversation,

(e.g, consider the strangeness of "Hello, this is Bill. Is this Mary?", outside a phone conversation)

and one of the conventions is that the introducer often says, of each introducee,

• It's (but almost never It is) <Insert Name>!

frequently adding phrases like And now; Appearing at the Palace nightly; The one, the only, etc.

I've always understood this as indicating the state/situation of that person being present.

"She's Paula Pountstone" means "That person(she) is Paula Poundstone."

"It's Paula Poundstone." means "The situation(it) is that Paula Poundstone is here."

• "She'll be performing Friday at the Comedy Club, it's Paula Poundstone."

In your context, the expression "it's Paula Poundstone" can be considered to be a truncated it-cleft. This usage is acceptable and has been for a long time. It is part of today's standard English.

The it-cleft's relative clause has been omitted, because its info is redundant and can be recovered from the context. The subject of the main clause of the it-cleft is the dummy pronoun "it". The dummy pronoun does not refer to anything or anybody: that is, it has no antecedent or referent.

The following is what your example basically means, with the omitted relative clause stuck back in using italics:

• "She'll be performing Friday at the Comedy Club, it's Paula Poundstone who will be performing Friday at the Comedy Club."

There are other threads here that are similar to yours. If you want more info or links to them, go ahead and ask.