Huh, this is what you call a mobile phone

Do you know the voice of that mobile phone is actually someone else's?

Voice library

Analysis

There'd be too much traffic if it was the raw voice, so a machine analyzes its characteristics and synthesizes the most similar one from the codebook, and sends it to the other side.

Is this it?

Uncountable voices are recorded in the codebook, it's like a voice library.

But Reimu doesn't have the voice of a middle-aged man?

The codebook only has two voices: Tesshō Genda and Akio Otsuka.

It's more like Pop Team Epic...