How to Build an On/Off Switch Triggered by Cellphone Microphone Signal?

Hi, sorry if what i am asking has already been answered somewhere in your forum and if i use the wrong terminology, but here's my problem:

I want to create an on/off switch, which will be on (circuit closed) when it receives an electric signal from a cellphone's microphone (meaning low voltage) and when it doesn't it will be off (circuit open).

So:
cellphone microphone on --> Circuit closed
cellphone microphone off --> circuit open

That's all. What do i need to buy and how do i build that?

Thanks a lot in advance!

I think what you want is a VOX operated switch
Lots of circuits on the net easily built with a op amp and 555 timer

http://www.electronickits.com/kit/complete/elec/k126.htm