Who was the caller in the episode “Last Call” (S03E15)

Question

In the Person of Interest episode S03E15, the POI was Sandra Nicholson, who works in a 911-dial in station. She was forced by a caller to delete calls from a certain date from the server.

At the end of the episode, the POI as well as the caller are alive. Finch gets a phone call from him that he will be after Finch

You, however, are another story entirely. Be seeing you.

Who was this caller? Was he ever mentioned after that? (Maybe I have missed something, but as of today's screened episodes, I have not seen him again).

Answer 1

The mysterious criminal mastermind known as The Voice has returned in the fifth season, in the Sotto Voce episode (S05E09).

A bit more detailed answer with spoilers:

The Voice is actually Terry Easton, the POI of the episode. Terry pretends to be a victim of The Voice and orchestrates an assassination of his former employee. Although The Voice escapes the police once again, he meets his end at the hands of Elias.