Wp8 an Android

hey, i have read a lot about mobile phones back-key. but when i try to acces it it doesnt work thats the code: //check if pause button (escape key) is pressed if(Input.GetKeyDown(KeyCode.Escape)){

i testet it on my lumia 920 32gb and it doesnt work. im going to test on android 4.2 jelly bean s3 mini

i use latest unity (4.3.xf)