side chain noise gate (dynamics processing)

Hi all,

I wonder if anyone can tell me if it is possible to set up dynamics processing as a noise gate which is side-chained to a vocal track such that the noise gate is ON when there is no signal from the vocal track sidechain and OFF when there IS a signal from the sidechain input.

I realize this is essentially a reverse noise gate!

What I'm trying to do is add a "telephone line noise" sample to a character voice in an audiobook production which is itself treated with EQ to give it a telphone-like sound, but I only want the "line noise" when the voice is speaking, not at any other time, e.g. when other characters or narrator are speaking.

I'm sure it's possible just can't get my head around how!